xfs: encode the rtbitmap in big endian format

xfs: encode the rtbitmap in big endian format

Source kernel commit: eba42c2c53c8b8905307b702c93dffef0719a896

Currently, the ondisk realtime bitmap file is accessed in units of
32-bit words.  There's no endian translation of the contents of this
file, which means that the Bad Things Happen(tm) if you go from (say)
x86 to powerpc.  Since we have a new feature flag, let's take the
opportunity to enforce an endianness on the file.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>