fs: add do_futimesat() helper; remove internal call to sys_futimesat()

Using this helper removes the in-kernel call to the sys_futimesat()
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: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>

fs/utimes.c

@@ -184,8 +184,8 @@ SYSCALL_DEFINE4(utimensat, int, dfd, const char __user *, filename,
 	return do_utimes(dfd, filename, utimes ? tstimes : NULL, flags);
 }
 
-SYSCALL_DEFINE3(futimesat, int, dfd, const char __user *, filename,
-		struct timeval __user *, utimes)
+static long do_futimesat(int dfd, const char __user *filename,
+			 struct timeval __user *utimes)
 {
 	struct timeval times[2];
 	struct timespec64 tstimes[2];
@@ -212,10 +212,17 @@ SYSCALL_DEFINE3(futimesat, int, dfd, const char __user *, filename,
 	return do_utimes(dfd, filename, utimes ? tstimes : NULL, 0);
 }
 
+
+SYSCALL_DEFINE3(futimesat, int, dfd, const char __user *, filename,
+		struct timeval __user *, utimes)
+{
+	return do_futimesat(dfd, filename, utimes);
+}
+
 SYSCALL_DEFINE2(utimes, char __user *, filename,
 		struct timeval __user *, utimes)
 {
-	return sys_futimesat(AT_FDCWD, filename, utimes);
+	return do_futimesat(AT_FDCWD, filename, utimes);
 }
 
 #ifdef CONFIG_COMPAT