Moved ckreads behind the LFS_M_CKREADS flag

Added some code, though we don't _really_ care:

           code          stack
  before: 37872           3048
  after:  38060 (+0.5%)   3056 (+0.3%)

Also interesting to note the difference in testing time, this highlights
_some_ of the performance cost of ckreads:

  with ckreads:    1135.92s
  without ckreads:  821.24s

lfs.h

@@ -155,6 +155,7 @@ enum lfs_type {
 #define LFS_M_RDWR               0  // Mount the filesystem as read and write
 #define LFS_M_RDONLY             1  // Mount the filesystem as read only
 #define LFS_M_CKPROGS   0x00000010  // Check progs by reading back progged data
+#define LFS_M_CKREADS   0x00000020  // Check reads via parity bits/checksums
 #define LFS_M_FLUSH     0x00000040  // Open all files with LFS_O_FLUSH
 #define LFS_M_SYNC      0x00000080  // Open all files with LFS_O_SYNC
 
@@ -176,6 +177,7 @@ enum lfs_type {
 // Filesystem info flags
 #define LFS_I_RDONLY    0x00000001  // Filesystem mounted read only
 #define LFS_I_CKPROGS   0x00000010  // Filesystem mounted with LFS_M_CKPROGS
+#define LFS_I_CKREADS   0x00000020  // Filesystem mounted with LFS_M_CKREADS
 #define LFS_I_FLUSH     0x00000040  // Filesystem mounted with LFS_M_FLUSH
 #define LFS_I_SYNC      0x00000080  // Filesystem mounted with LFS_M_SYNC