kernel: add ksys_unshare() helper; remove in-kernel calls to sys_unshare()

Using this helper allows us to avoid the in-kernel calls to the
sys_unshare() 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_unshare().

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>
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>

kernel/fork.c

@@ -2354,7 +2354,7 @@ static int unshare_fd(unsigned long unshare_flags, struct files_struct **new_fdp
  * constructed. Here we are modifying the current, active,
  * task_struct.
  */
-SYSCALL_DEFINE1(unshare, unsigned long, unshare_flags)
+int ksys_unshare(unsigned long unshare_flags)
 {
 	struct fs_struct *fs, *new_fs = NULL;
 	struct files_struct *fd, *new_fd = NULL;
@@ -2470,6 +2470,11 @@ bad_unshare_out:
 	return err;
 }
 
+SYSCALL_DEFINE1(unshare, unsigned long, unshare_flags)
+{
+	return ksys_unshare(unshare_flags);
+}
+
 /*
  *	Helper to unshare the files of the current task.
  *	We don't want to expose copy_files internals to