A user reported an error where we hit an assertion due to deleting a key
in an internal snapshot node, when deleting a dirent that points to a
nonexisting inode.

We try to avoid doing updates to keys for internal snapshot nodes, but
upon inspection of the places where we remove dirents in fsck it appears
BTREE_UPDATE_INTERNAL_SNAPSHOT_NODE is correct for all of them: either
the target dirent doesn't exist, or it's a directory with multiple
dirents pointing to it.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>