tune2fs: reset MMP state on error exit

tune2fs: reset MMP state on error exit

If tune2fs cannot perform the requested change, ensure that the MMP
block is reset to the unused state before exiting.  Otherwise, the
filesystem will be left with mmp_seq = EXT4_MMP_SEQ_FSCK set, which
prevents it from being mounted afterward:

    EXT4-fs warning (device dm-9): ext4_multi_mount_protect:311:
        fsck is running on the filesystem

Add a test to try some failed tune2fs operations and verify that the
MMP block is left in a clean state afterward.

Lustre-bug-id: https://jira.whamcloud.com/browse/LU-13672
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>