fs: add ksys_fallocate() wrapper; remove in-kernel calls to sys_fallocate()

Using the ksys_fallocate() wrapper allows us to get rid of in-kernel
calls to the sys_fallocate() 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_fallocate().

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 91e85e2..0779d47 100644
--- a/arch/mips/kernel/linux32.c
+++ b/arch/mips/kernel/linux32.c
@@ -157,6 +157,6 @@
 asmlinkage long sys32_fallocate(int fd, int mode, unsigned offset_a2,
 	unsigned offset_a3, unsigned len_a4, unsigned len_a5)
 {
-	return sys_fallocate(fd, mode, merge_64(offset_a2, offset_a3),
-			     merge_64(len_a4, len_a5));
+	return ksys_fallocate(fd, mode, merge_64(offset_a2, offset_a3),
+			      merge_64(len_a4, len_a5));
 }