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>
diff --git a/arch/mips/kernel/linux32.c b/arch/mips/kernel/linux32.c
index 58e7dd27..91e85e2 100644
--- a/arch/mips/kernel/linux32.c
+++ b/arch/mips/kernel/linux32.c
@@ -105,13 +105,13 @@ SYSCALL_DEFINE5(32_llseek, unsigned int, fd, unsigned int, offset_high,
 SYSCALL_DEFINE6(32_pread, unsigned long, fd, char __user *, buf, size_t, count,
 	unsigned long, unused, unsigned long, a4, unsigned long, a5)
 {
-	return sys_pread64(fd, buf, count, merge_64(a4, a5));
+	return ksys_pread64(fd, buf, count, merge_64(a4, a5));
 }
 
 SYSCALL_DEFINE6(32_pwrite, unsigned int, fd, const char __user *, buf,
 	size_t, count, u32, unused, u64, a4, u64, a5)
 {
-	return sys_pwrite64(fd, buf, count, merge_64(a4, a5));
+	return ksys_pwrite64(fd, buf, count, merge_64(a4, a5));
 }
 
 SYSCALL_DEFINE1(32_personality, unsigned long, personality)