e2fsck: regression tests for INCOMPAT_MMP feature

e2fsck: regression tests for INCOMPAT_MMP feature

Add tests for the MMP feature - creating a filesystem with mke2fs
and MMP enabled, enable/disable MMP with tune2fs, disabling the
e2fsck MMP flag with tune2fs after a failed e2fsck, and e2fsck
checking and fixing a corrupt MMP block.

The MMP tests need to be run from a real disk, not tmpfs, because
tmpfs doesn't support O_DIRECT reads, which MMP uses to ensure
that reads from the MMP block are not filled from the page cache.
Using a local disk does not slow down the tests noticably, since
they wait to detect if the MMP block is being modified.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>