btrfs-progs: fix unaligned compat endian warnings

btrfs-progs: fix unaligned compat endian warnings

The _una_ struct's entire job is to pass an argument to le*_to_cpu.  So
it's a little embarassing that it uses a native cpu types and generates
endian warnings.

ctree.h:1616:1: warning: incorrect type in assignment (different base types)
ctree.h:1616:1:    expected unsigned long long [unsigned] [usertype] x
ctree.h:1616:1:    got restricted __le64 [usertype] <noident>

Signed-off-by: Zach Brown <zab@redhat.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>