Drop local definition of MOUNT_BUSY

It's accessible via <sys/mntent.h>.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tom Caputi <tcaputi@datto.com>
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@osnexus.com>
Closes openzfs#8765

module/os/linux/zfs/zfs_ctldir.c

@@ -85,6 +85,7 @@
 #include <sys/dsl_destroy.h>
 #include <sys/dsl_deleg.h>
 #include <sys/zpl.h>
+#include <sys/mntent.h>
 #include "zfs_namecheck.h"
 
 /*
@@ -1047,8 +1048,6 @@ zfsctl_snapshot_unmount(char *snapname, int flags)
 	return (error);
 }
 
-#define	MOUNT_BUSY 0x80		/* Mount failed due to EBUSY (from mntent.h) */
-
 int
 zfsctl_snapshot_mount(struct path *path, int flags)
 {