btrfs: use PTR_ERR() instead of PTR_ERR_OR_ZERO() for btrfs_get_extent()

The function btrfs_get_extent() will only return an PTR_ERR() or a valid
extent map pointer. It will not return NULL.

Thus the usage of PTR_ERR_OR_ZERO() inside submit_one_sector() is not
needed, use plain PTR_ERR() instead, and that is the only usage of
PTR_ERR_OR_ZERO() after btrfs_get_extent().

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>