Add multi-mount protection support to libext2fs (INCOMPAT_MMP feature).

This allows mke2fs, e2fsck, and others to detect if the filesystem is
mounted on a remote node (on SAN disks) and avoid corrupting the
filesystem.  For e2fsprogs this only means that it check the MMP block
to see if the filesystem is in use, and mark the filesystem busy while
e2fsck is running on the system.

There is no requirement that e2fsck updates the MMP block in any regular
interval, but e2fsck does this occasionally to provide additional
information to the sysadmin in case of conflict.

Signed-off-by: Kalpak Shah <kalpak.shah@sun.com>
Signed-off-by: Andreas Dilger <adilger@sun.com>