btrfs: Fix compilation on big endian systems

btrfs: Fix compilation on big endian systems

Fix following two compile errors on big endian systems:

  CC      fs/btrfs/btrfs.o
In file included from include/linux/byteorder/big_endian.h:107,
                 from ./arch/powerpc/include/asm/byteorder.h:82,
                 from ./arch/powerpc/include/asm/bitops.h:8,
                 from include/linux/bitops.h:152,
                 from include/uuid.h:9,
                 from fs/btrfs/btrfs.c:10:
fs/btrfs/conv-funcs.h: In function ‘btrfs_key_to_disk’:
include/linux/byteorder/generic.h:90:21: error: ‘__cpu_to_le16’ undeclared (first use in this function); did you mean ‘__cpu_to_le16p’?
 #define cpu_to_le16 __cpu_to_le16
                     ^~~~~~~~~~~~~
fs/btrfs/conv-funcs.h:79:10: note: in expansion of macro ‘cpu_to_le16’
   __u16: cpu_to_le16,     \
          ^~~~~~~~~~~

  CC      fs/btrfs/compression.o
In file included from ./arch/powerpc/include/asm/unaligned.h:9,
                 from fs/btrfs/compression.c:16:
include/linux/unaligned/access_ok.h:6:19: error: redefinition of ‘get_unaligned_le16’
 static inline u16 get_unaligned_le16(const void *p)
                   ^~~~~~~~~~~~~~~~~~
In file included from fs/btrfs/ctree.h:16,
                 from fs/btrfs/btrfs.h:12,
                 from fs/btrfs/compression.c:8:
include/linux/unaligned/le_byteshift.h:40:19: note: previous definition of ‘get_unaligned_le16’ was here
 static inline u16 get_unaligned_le16(const void *p)
                   ^~~~~~~~~~~~~~~~~~

Include file asm/unaligned.h contains arch specific macros and functions
for unaligned access as opposite to linux/unaligned le_byteshift.h which
contains macros and functions specific to little endian systems only.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Qu Wenruo <wqu@suse.com>