Fix btrfs bus error on sparc on snapshot delete

Fix btrfs bus error on sparc on snapshot delete

The following patch fixes memory alignment and endianness
issue while doing a snapshot deletion with btrfs as a
backing store on platform such as sparc.

The implementation is taken from btrfs-progs.

Changes since v1:
- include <byteswap.h> for bswap definition
- include defined function name as a comment above BTRFS_SETGET_STACK_FUNCS

Signed-off-by: Thomas Tanaka <thomas.tanaka@oracle.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>