mm: add ksys_readahead() helper; remove in-kernel calls to sys_readahead()

Using this helper allows us to avoid the in-kernel calls to the
sys_readahead() 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_readahead().

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: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-mm@kvack.org
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>

arch/parisc/kernel/sys_parisc.c

@@ -345,7 +345,7 @@ asmlinkage ssize_t parisc_pwrite64(unsigned int fd, const char __user *buf,
 asmlinkage ssize_t parisc_readahead(int fd, unsigned int high, unsigned int low,
 		                    size_t count)
 {
-	return sys_readahead(fd, (loff_t)high << 32 | low, count);
+	return ksys_readahead(fd, (loff_t)high << 32 | low, count);
 }
 
 asmlinkage long parisc_fadvise64_64(int fd,