libext2fs: sort keys for xattr blocks

libext2fs: sort keys for xattr blocks

Richard Purdie reports that libext2fs doesn't sort attribute keys in
the xattr block correctly, causing the kernel to return -ENODATA when
querying attributes that should be there.  Therefore, sort attributes
so that whatever ends up in the xattr block is sorted according to
what the kernel expects.

Cc: Darren Hart <dvhart@linux.intel.com>
Reported-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Tested-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>