ext4: use 64-bit timestamps for mmp_time

The mmp_time field is 64 bits wide, which is good, but calling
get_seconds() results in a 32-bit value on 32-bit architectures. Using
ktime_get_real_seconds() instead returns 64 bits everywhere.

Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>