Implement relatime.

Add the "relatime" property.  When set to "on", a file's atime will only
be updated if the existing atime at least a day old or if the existing
ctime or mtime has been updated since the last access.  This behavior
is compatible with the Linux "relatime" mount option.

Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2064
Closes #1917

include/sys/fs/zfs.h

@@ -147,6 +147,7 @@ typedef enum {
 	ZFS_PROP_SELINUX_FSCONTEXT,
 	ZFS_PROP_SELINUX_DEFCONTEXT,
 	ZFS_PROP_SELINUX_ROOTCONTEXT,
+	ZFS_PROP_RELATIME,
 	ZFS_NUM_PROPS
 } zfs_prop_t;
 

include/sys/zfs_vfsops.h

@@ -65,6 +65,7 @@ typedef struct zfs_sb {
 	boolean_t	z_utf8;		/* utf8-only */
 	int		z_norm;		/* normalization flags */
 	boolean_t	z_atime;	/* enable atimes mount option */
+	boolean_t	z_relatime;	/* enable relatime mount option */
 	boolean_t	z_unmounted;	/* unmounted */
 	rrwlock_t	z_teardown_lock;
 	krwlock_t	z_teardown_inactive_lock;