btrfs-progs: Stop stomping on 'name' input parameter

In btrfs_name_hash, Local variable 'buf' is declared as

	__u32 buf[2];

but we then try to do this:

	buf[0] = 0x67452301;
	buf[1] = 0xefcdab89;
	buf[2] = 0x98badcfe;
	buf[3] = 0x10325476;

Oops. Fix buf to be the proper size.

Signed-off-by: Alex Chiang <achiang@hp.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>