btrfs: add strict extent map alignment checks

btrfs: add strict extent map alignment checks

Currently we do not check the alignment of extent_map structure.

The reasons are the inode and extent-map tests use unaligned values
for start offsets and lengths.

Thankfully those legacy problems are properly addressed by previous
patches, now we can finally put the alignment checks into
validate_extent_map().

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