fs: add ksys_p{read,write}64() helpers; remove in-kernel calls to syscalls

Using the ksys_p{read,write}64() wrappers allows us to get rid of
in-kernel calls to the sys_pread64() and sys_pwrite64() syscalls.
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_p{read,write}64().

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>

include/linux/syscalls.h

@@ -965,6 +965,10 @@ int ksys_unshare(unsigned long unshare_flags);
 int ksys_setsid(void);
 int ksys_sync_file_range(int fd, loff_t offset, loff_t nbytes,
 			 unsigned int flags);
+ssize_t ksys_pread64(unsigned int fd, char __user *buf, size_t count,
+		     loff_t pos);
+ssize_t ksys_pwrite64(unsigned int fd, const char __user *buf,
+		      size_t count, loff_t pos);
 
 /*
  * The following kernel syscall equivalents are just wrappers to fs-internal