libext2fs: fix ext2fs_close() when MMP is not enabled

libext2fs: fix ext2fs_close() when MMP is not enabled

If MMP support is not configured, then ext2fs_mmp_stop() will always
return the error EXT2_ET_OP_NOT_SUPPORTED.  Unfortunately,
ext2fs_close() and tune2fs call ext2fs_mmp_stop() unconditionally.  So
if the file system does not have MMP enabled, fix ext2fs_mmp_stop() to
return success even if CONFIG_MMP is not enabled, so that
ext2fs_close() and tune2fs doesn't fail for no good reason.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>