xfs: Use basic types to define xfs_log_dinode's di_nextents and di_anextents

A future commit will increase the width of xfs_extnum_t in order to facilitate
larger per-inode extent counters. Hence this patch now uses basic types to
define xfs_log_dinode->[di_nextents|dianextents].

Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Chandan Babu R <chandan.babu@oracle.com>