Linux 4.18 compat: inode timespec -> timespec64

Commit torvalds/linux@95582b0 changes the inode i_atime, i_mtime,
and i_ctime members form timespec's to timespec64's to make them
2038 safe.  As part of this change the current_time() function was
also updated to return the timespec64 type.

Resolve this issue by introducing a new inode_timespec_t type which
is defined to match the timespec type used by the inode.  It should
be used when working with inode timestamps to ensure matching types.

The timestruc_t type under Illumos was used in a similar fashion but
was specified to always be a timespec_t.  Rather than incorrectly
define this type all timespec_t types have been replaced by the new
inode_timespec_t type.

Finally, the kernel and user space 'sys/time.h' headers were aligned
with each other.  They define as appropriate for the context several
constants as macros and include static inline implementation of
gethrestime(), gethrestime_sec(), and gethrtime().

Reviewed-by: Chunwei Chen <tuxoko@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7643

include/sys/dsl_dir.h

@@ -108,7 +108,7 @@ struct dsl_dir {
 	/* Protected by dd_lock */
 	kmutex_t dd_lock;
 	list_t dd_props; /* list of dsl_prop_record_t's */
-	timestruc_t dd_snap_cmtime; /* last time snapshot namespace changed */
+	inode_timespec_t dd_snap_cmtime; /* last snapshot namespace change */
 	uint64_t dd_origin_txg;
 
 	/* gross estimate of space used by in-flight tx's */
@@ -180,7 +180,7 @@ boolean_t dsl_dir_is_clone(dsl_dir_t *dd);
 void dsl_dir_new_refreservation(dsl_dir_t *dd, struct dsl_dataset *ds,
     uint64_t reservation, cred_t *cr, dmu_tx_t *tx);
 void dsl_dir_snap_cmtime_update(dsl_dir_t *dd);
-timestruc_t dsl_dir_snap_cmtime(dsl_dir_t *dd);
+inode_timespec_t dsl_dir_snap_cmtime(dsl_dir_t *dd);
 void dsl_dir_set_reservation_sync_impl(dsl_dir_t *dd, uint64_t value,
     dmu_tx_t *tx);
 void dsl_dir_zapify(dsl_dir_t *dd, dmu_tx_t *tx);