bcachefs: Kill snapshot arg to fsck_write_inode()

It was initially believed that it would be better to be explicit about
the snapshot we're updating when writing inodes in fsck; however, it
turns out that passing around the snapshot separately is more error
prone and we're usually updating the inode in the same snapshow we read
it from.

This is different from normal filesystem paths, where we do the update
in the snapshot of the subvolume we're in.

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