I was hitting weird issues when trying to remove hole extents and it turned out
it was because I was sending non-aligned offsets down to
btrfs_lookup_csums_range.  So add an assert for this in case somebody trips over
this in the future.  Thanks,
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>