OpenZFS 7336 - vfork and O_CLOEXEC causes zfs_mount EBUSY

Porting notes:
- statvfs64 is replaced by statfs64.
- ZFS_SUPER_MAGIC definition moved in include/sys/fs/zfs.h
  to share it between user and kernel space.

Authored by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: Robert Mustacchi <rm@joyent.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: George Melikov <mail@gmelikov.ru>

OpenZFS-issue: https://www.illumos.org/issues/7336
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/dd862f6d
Closes #5651

include/sys/fs/zfs.h

@@ -924,6 +924,8 @@ typedef struct ddt_histogram {
 #define	ZFS_DEV		"/dev/zfs"
 #define	ZFS_SHARETAB	"/etc/dfs/sharetab"
 
+#define	ZFS_SUPER_MAGIC	0x2fc12fc1
+
 /* general zvol path */
 #define	ZVOL_DIR	"/dev"
 

include/sys/zfs_vfsops.h

@@ -119,8 +119,6 @@ typedef struct zfs_sb {
 	kmutex_t	*z_hold_locks;	/* znode hold locks */
 } zfs_sb_t;
 
-#define	ZFS_SUPER_MAGIC	0x2fc12fc1
-
 #define	ZSB_XATTR	0x0001		/* Enable user xattrs */
 
 /*