attrs: Dropped lfsr_setattr flags

After running into issues with LFS_A_CREAT/EXCL in file-attached custom
attributes, we're left in a really weird place:

- None of lfs_setattr's flags are valid in lfs_attr
- None of lfs_attr's flags are valid in lfs_setattr

I also started thinking about the actual use case for LFS_A_CREAT/EXCL,
and it's really not clear.

littlefs really doesn't care about interprocess communication the same
way POSIX/other filesystem APIs do. We can always rely on integration
layers wrapping up multiple operations in a single mutex, so offering
flexible creation semantics has diminished value. LFS_A_CREAT and
LFS_A_EXCL can both be emulated by calling lfsr_getattr first and
checking its return value.

Thinking ahead to the hypothetical lfsr_set API. The main purpose of
lfsr_set is to provide an API that's easier to use but less powerful
than lfsr_file_open. And adding a flags argument seems to run counter to
that.

For example, if you saw this code with no knowledge of littlefs:

  lfsr_setattr(&lfs, "cat", 'a', "meow", 4, 0);

You would probably be surprised that it returns LFS_ERR_NOENT without
additional flags.

I realize Linux sidesteps this with XATTR_CREATE/REPLACE by making 0
default to implicitly creating, but I didn't want to introduce
inconsistent flag behavior like this unless I had to.

---

So for now dropping LFS_A_CREAT/EXCL and flags argument to lfsr_setattr.

Code savings minimal, this was mostly for API ergonomics:

           code          stack
  before: 38104           2624
  after:  38084 (-0.1%)   2624 (+0.0%)

lfs.h

@@ -155,9 +155,7 @@ enum lfs_type {
 #define LFS_A_RDONLY             0  // Open an attr as read only
 #define LFS_A_WRONLY             1  // Open an attr as write only
 #define LFS_A_RDWR               2  // Open an attr as read and write
-#define LFS_A_CREAT           0x04  // Create an attr if it does not exist
-#define LFS_A_EXCL            0x08  // Fail if an attr already exists
-#define LFS_A_LAZY            0x10  // Only write attr if file changed
+#define LFS_A_LAZY            0x04  // Only write attr if file changed
 
 // Filesystem format flags
 #define LFS_F_RDWR               0  // Format the filesystem as read and write
@@ -951,17 +949,13 @@ lfs_ssize_t lfsr_getattr(lfs_t *lfs, const char *path, uint8_t type,
 lfs_ssize_t lfsr_sizeattr(lfs_t *lfs, const char *path, uint8_t type);
 
 #ifndef LFS_READONLY
-// Set custom attributes
-//
-// The flags field controls the exact behavior if the attribute is or
-// isn't found.
+// Set a custom attributes
 //
 // Returns a negative error code on failure.
 //int lfs_setattr(lfs_t *lfs, const char *path,
 //        uint8_t type, const void *buffer, lfs_size_t size);
 int lfsr_setattr(lfs_t *lfs, const char *path, uint8_t type,
-        const void *buffer, lfs_size_t size,
-        uint32_t flags);
+        const void *buffer, lfs_size_t size);
 #endif
 
 #ifndef LFS_READONLY