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define this type all timespec_t types have been replaced by the new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/zfs_context.h

@@ -483,7 +483,7 @@ extern char *vn_dumpdir;
 #define	AV_SCANSTAMP_SZ	32		/* length of anti-virus scanstamp */
 
 typedef struct xoptattr {
-	timestruc_t	xoa_createtime;	/* Create time of file */
+	inode_timespec_t xoa_createtime;	/* Create time of file */
 	uint8_t		xoa_archive;
 	uint8_t		xoa_system;
 	uint8_t		xoa_readonly;
@@ -596,13 +596,6 @@ extern void delay(clock_t ticks);
 #define	USEC_TO_TICK(usec)	((usec) / (MICROSEC / hz))
 #define	NSEC_TO_TICK(usec)	((usec) / (NANOSEC / hz))
 
-#define	gethrestime_sec() time(NULL)
-#define	gethrestime(t) \
-	do {\
-		(t)->tv_sec = gethrestime_sec();\
-		(t)->tv_nsec = 0;\
-	} while (0);
-
 #define	max_ncpus	64
 #define	boot_ncpus	(sysconf(_SC_NPROCESSORS_ONLN))