bcachefs: fsck: Break walk_inode() up into multiple functions

bcachefs: fsck: Break walk_inode() up into multiple functions

Some refactoring, prep work for algorithm improvements related to
snapshots.

we need to add a bitmap to the list of inodes for "seen this snapshot";
for this bitmap to correctly be available, we'll need to gather the list
of inodes first, and later look up the inode for a given snapshot.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>