gc: Renamed lfsr_gc -> lfsr_fs_gc, keep lfsr_fs_unck in non-gc

- lfsr_gc -> lfsr_fs_gc
- lfsr_gc_unck -> lfsr_fs_unck

lfsr_fs_unck is surprisingly still useful in non-gc builds, since we
still have ckmeta/ckdata state. These flags can still be queried with
lfsr_fs_stat and cleared with lfsr_fs_ckmeta/ckdata/lfsr_traversal_t, so
it seems useful to keep this function around.

It's also a relatively cheap function.

Though this does mean it deserves a rename. Dropping the gc prefix
hopefully makes it clearer this function is not entirely gc-specific.

And since we no longer have lfsr_gc_setflags/setsteps, it makes sense to
rename lfsr_gc back to lfsr_fs_gc, to be consistent with the other
filesystem-wide utilities.

Code changes, apparently lfsr_fs_unck costs 12 bytes:

                   code          stack          ctx
  default before: 37792           2608          620
  default after:  37804 (+0.0%)   2608 (+0.0%)  620 (+0.0%)

  gc before:      37938           2608          768
  gc after:       37940 (+0.0%)   2608 (+0.0%)  768 (+0.0%)

tests/test_ck.toml

@@ -8,7 +8,7 @@ after = ['test_traversal', 'test_gc', 'test_mount']
 # test we can detect at least fully clobbered blocks
 [cases.test_ck_ckmeta_easy]
 # METHOD=0 => lfsr_fs_ckmeta
-# METHOD=1 => lfsr_gc
+# METHOD=1 => lfsr_fs_gc
 # METHOD=2 => lfsr_traversal_read
 # METHOD=3 => lfsr_mount
 defines.METHOD = [0, 1, 2, 3]
@@ -99,10 +99,10 @@ code = '''
         if (METHOD == 0) {
             lfsr_fs_ckmeta(&lfs) => LFS_ERR_CORRUPT;
 
-        // find clobbered blocks with lfsr_gc
+        // find clobbered blocks with lfsr_fs_gc
         } else if (METHOD == 1) {
             #ifdef LFS_GC
-            lfsr_gc(&lfs) => LFS_ERR_CORRUPT;
+            lfsr_fs_gc(&lfs) => LFS_ERR_CORRUPT;
             #else
             LFS_UNREACHABLE();
             #endif
@@ -145,7 +145,7 @@ done:;
 
 [cases.test_ck_ckdata_easy]
 # METHOD=0 => lfsr_fs_ckdata
-# METHOD=1 => lfsr_gc
+# METHOD=1 => lfsr_fs_gc
 # METHOD=2 => lfsr_traversal_read
 # METHOD=3 => lfsr_mount
 defines.METHOD = [0, 1, 2, 3]
@@ -237,10 +237,10 @@ code = '''
         if (METHOD == 0) {
             lfsr_fs_ckdata(&lfs) => LFS_ERR_CORRUPT;
 
-        // find clobbered blocks with lfsr_gc
+        // find clobbered blocks with lfsr_fs_gc
         } else if (METHOD == 1) {
             #ifdef LFS_GC
-            lfsr_gc(&lfs) => LFS_ERR_CORRUPT;
+            lfsr_fs_gc(&lfs) => LFS_ERR_CORRUPT;
             #else
             LFS_UNREACHABLE();
             #endif