libext2fs: Always swab the MMP block on big-endian systems machines

libext2fs: Always swab the MMP block on big-endian systems machines

The MMP code in libext2fs tries to gate MMP block swab'ing with this
test:

if (fs->super->s_magic == ext2fs_swab16(EXT2_SUPER_MAGIC))

However, EXT2FS_ENABLE_SWAPFS never seems to be defined anywhere (all
possible existed, the field fs->super->s_magic is always in host
byteorder, so the test always fails.  So, we can change the #ifdef to
WORDS_BIGENDIAN (which is conditionally defined on BE platforms) and
get rid of the broken if test.

Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>