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.c

@@ -11210,13 +11210,7 @@ lfs_ssize_t lfsr_sizeattr(lfs_t *lfs, const char *path, uint8_t type) {
 }
 
 int lfsr_setattr(lfs_t *lfs, const char *path, uint8_t type,
-        const void *buffer, lfs_size_t size,
-        uint32_t flags) {
-    // unknown flags?
-    LFS_ASSERT((flags & ~(
-            LFS_A_CREAT
-                | LFS_O_EXCL)) == 0);
-
+        const void *buffer, lfs_size_t size) {
     // prepare our filesystem for writing
     int err = lfsr_fs_mkconsistent(lfs);
     if (err) {
@@ -11232,18 +11226,6 @@ int lfsr_setattr(lfs_t *lfs, const char *path, uint8_t type,
         return err;
     }
 
-    // doesn't exist?
-    if (!lfsr_o_iscreat(flags)
-            && err == LFS_ERR_NOATTR) {
-        return LFS_ERR_NOATTR;
-
-    // does exist?
-    } else if (lfsr_o_iscreat(flags)
-            && lfsr_o_isexcl(flags)
-            && err != LFS_ERR_NOATTR) {
-        return LFS_ERR_EXIST;
-    }
-
     // commit our attr
     lfs_alloc_ckpoint(lfs);
     err = lfsr_mdir_commit(lfs, &mdir, LFSR_ATTRS(