tune2fs: check return value of ext2fs_mmp_update2 in rewrite_metadata_checksums

tune2fs: check return value of ext2fs_mmp_update2 in rewrite_metadata_checksums

Tune2fs hasn't consider about the result of executing ext2fs_mmp_update2
when it try to rewrite_metadata_checksums. If the ext2fs_mmp_update2
failed, multi-mount protection couldn't guard there has the only node
(i.e. this program) accessing this device in the meantime.

We solve this problem to verify the return value of ext2fs_mmp_update2.
It terminate rewrite_metadata_checksums and exit immediately if the
wrong error code returned.

Signed-off-by: lihaoxiang <lihaoxiang9@huawei.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>