btrfs: remove checks for a 0 inode number during backref walking

When doing backref walking to determine if an extent is shared, we are
testing if the inode number, stored in the 'inum' field of struct
share_check, is 0. However that can never be case, since the all instances
of the structure are created at btrfs_is_data_extent_shared(), which
always initializes it with the inode number from a fs tree (and the number
for any inode from any tree can never be 0). So remove the checks.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>