Regardless of whether the caller is interested or not in knowing the inode's
generation (dir_gen != NULL), get_first_ref always does a btree lookup to get
the inode item. Avoid this useless lookup if dir_gen parameter is NULL (which
is in some cases).
Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
Signed-off-by: Chris Mason <clm@fb.com>