fs: add ksys_unlink() wrapper; remove in-kernel calls to sys_unlink()

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

In the near future, all callers of ksys_unlink() should be converted to
call do_unlinkat() directly or, at least, to operate on regular kernel
pointers.

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_rd.c

@@ -288,7 +288,7 @@ noclose_input:
 	sys_close(out_fd);
 out:
 	kfree(buf);
-	sys_unlink("/dev/ram");
+	ksys_unlink("/dev/ram");
 	return res;
 }