fs: add ksys_open() wrapper; remove in-kernel calls to sys_open()

Using this wrapper allows us to avoid the in-kernel calls to the
sys_open() syscall. The ksys_ prefix denotes that this function is meant
as a drop-in replacement for the syscall. In particular, it uses the
same calling convention as sys_open().

This patch is part of a series which removes in-kernel calls to syscalls.
On this basis, the syscall entry path can be streamlined. For details, see
http://lkml.kernel.org/r/20180325162527.GA17492@light.dominikbrodowski.net

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>

init/do_mounts_md.c

@@ -181,7 +181,7 @@ static void __init md_setup_drive(void)
 			partitioned ? "_d" : "", minor,
 			md_setup_args[ent].device_names);
 
-		fd = sys_open(name, 0, 0);
+		fd = ksys_open(name, 0, 0);
 		if (fd < 0) {
 			printk(KERN_ERR "md: open failed - cannot start "
 					"array %s\n", name);
@@ -244,7 +244,7 @@ static void __init md_setup_drive(void)
 			 * array without it
 			 */
 			ksys_close(fd);
-			fd = sys_open(name, 0, 0);
+			fd = ksys_open(name, 0, 0);
 			sys_ioctl(fd, BLKRRPART, 0);
 		}
 		ksys_close(fd);
@@ -294,7 +294,7 @@ static void __init autodetect_raid(void)
 
 	wait_for_device_probe();
 
-	fd = sys_open("/dev/md0", 0, 0);
+	fd = ksys_open("/dev/md0", 0, 0);
 	if (fd >= 0) {
 		sys_ioctl(fd, RAID_AUTORUN, raid_autopart);
 		ksys_close(fd);