Added lfsr_file_ckmeta/ckdata

These are basically the same as lfsr_fs_ckmeta/ckdata but limited to a
single file. They may be useful when you need to validate a file but
don't want to bother validating the entire filesystem:

  // Check a file for metadata errors
  int lfsr_file_ckmeta(lfs_t *lfs, lfsr_file_t *file);

  // Check a file for metadata + data errors
  int lfsr_file_ckdata(lfs_t *lfs, lfsr_file_t *file);

I've also added test_ck to test these and added some more
lfsr_fs_ckmeta/ckdata tests there. These currently just test simple
full-block clobbering, but we should eventually test more interesting
error patterns.

Unfortunately lfsr_file_ckmeta/ckdata can't reuse the internal
lfsr_mtree_traverse in quite the same way lfsr_fs_ckmeta/ckdata can, so
they're actually a bit more expensive. Though keep in mind with
link-time gc you won't pay the cost unless you call these functions:

           code          stack
  before: 36024           2696
  after:  36368 (+1.0%)   2664 (-1.2%)

Oh, and the multiple calls to lfsr_btree/bshrub_traverse apparently
uninlined it out of lfsr_mtree_traverse, saving the stack cost in the
stack hot-path... Yay?

tests/test_mount.toml

@@ -237,6 +237,8 @@ code = '''
 '''
 
 # test we can detect at least fully clobbered blocks
+#
+# these are tested more thoroughly in test_ck
 [cases.test_mount_t_ckmeta]
 defines.N = [1, 2, 4, 8, 16, 32, 64]
 defines.SIZE = [