In get_extent_skip_holes() we're checking the return of
btrfs_get_extent_fiemap() for an error pointer or NULL, but
btrfs_get_extent_fiemap() will never return NULL, only error pointers or
a valid extent_map.

The other caller of btrfs_get_extent_fiemap(), find_desired_extent(),
correctly only checks for error-pointers.
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>