fs: add do_compat_fcntl64() helper; remove in-kernel call to compat syscall

Using the fs-internal do_compat_fcntl64() helper allows us to get rid of
the fs-internal call to the compat_sys_fcntl64() syscall.

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>

fs/fcntl.c

@@ -607,8 +607,8 @@ static int fixup_compat_flock(struct flock *flock)
 	return 0;
 }
 
-COMPAT_SYSCALL_DEFINE3(fcntl64, unsigned int, fd, unsigned int, cmd,
-		       compat_ulong_t, arg)
+static long do_compat_fcntl64(unsigned int fd, unsigned int cmd,
+			     compat_ulong_t arg)
 {
 	struct fd f = fdget_raw(fd);
 	struct flock flock;
@@ -672,6 +672,12 @@ COMPAT_SYSCALL_DEFINE3(fcntl64, unsigned int, fd, unsigned int, cmd,
 	return err;
 }
 
+COMPAT_SYSCALL_DEFINE3(fcntl64, unsigned int, fd, unsigned int, cmd,
+		       compat_ulong_t, arg)
+{
+	return do_compat_fcntl64(fd, cmd, arg);
+}
+
 COMPAT_SYSCALL_DEFINE3(fcntl, unsigned int, fd, unsigned int, cmd,
 		       compat_ulong_t, arg)
 {
@@ -684,7 +690,7 @@ COMPAT_SYSCALL_DEFINE3(fcntl, unsigned int, fd, unsigned int, cmd,
 	case F_OFD_SETLKW:
 		return -EINVAL;
 	}
-	return compat_sys_fcntl64(fd, cmd, arg);
+	return do_compat_fcntl64(fd, cmd, arg);
 }
 #endif