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

Using the fs-internal do_lookup_dcookie() helper allows us to get rid of
fs-internal calls to the sys_lookup_dcookie() 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/dcookies.c

@@ -146,7 +146,7 @@ out:
 /* And here is where the userspace process can look up the cookie value
  * to retrieve the path.
  */
-SYSCALL_DEFINE3(lookup_dcookie, u64, cookie64, char __user *, buf, size_t, len)
+static int do_lookup_dcookie(u64 cookie64, char __user *buf, size_t len)
 {
 	unsigned long cookie = (unsigned long)cookie64;
 	int err = -EINVAL;
@@ -203,13 +203,18 @@ out:
 	return err;
 }
 
+SYSCALL_DEFINE3(lookup_dcookie, u64, cookie64, char __user *, buf, size_t, len)
+{
+	return do_lookup_dcookie(cookie64, buf, len);
+}
+
 #ifdef CONFIG_COMPAT
 COMPAT_SYSCALL_DEFINE4(lookup_dcookie, u32, w0, u32, w1, char __user *, buf, compat_size_t, len)
 {
 #ifdef __BIG_ENDIAN
-	return sys_lookup_dcookie(((u64)w0 << 32) | w1, buf, len);
+	return do_lookup_dcookie(((u64)w0 << 32) | w1, buf, len);
 #else
-	return sys_lookup_dcookie(((u64)w1 << 32) | w0, buf, len);
+	return do_lookup_dcookie(((u64)w1 << 32) | w0, buf, len);
 #endif
 }
 #endif