mmp: fix 64-bit handling of s_mmp_block

mmp: fix 64-bit handling of s_mmp_block

Fix the checking of s_mmp_block in e2fsck_pass1() and
ext2fs_mmp_read() to handle the high 32 bits of s_blocks_count.
Remove redundant check of s_mmp_block in do_dump_mmp() right before
ext2fs_mmp_read() is called.

Also fix s_blocks_count_hi in check_backup_super_block(), since it
cannot use the ext2fs_blocks_count() helper easily.

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