ext4: don't leave i_crtime.tv_sec uninitialized

If the i_crtime field is not present in the inode, don't leave the
field uninitialized.

Fixes: ef7f3835

 ("ext4: Add nanosecond timestamps")
Reported-by: Vegard Nossum <vegard.nossum@oracle.com>
Tested-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: stable@vger.kernel.org