fs: add ksys_read() helper; remove in-kernel calls to sys_read()

Using this helper allows us to avoid the in-kernel calls to the
sys_read() 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_read().

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: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>
diff --git a/init/do_mounts.c b/init/do_mounts.c
index b17e009..2c71dab 100644
--- a/init/do_mounts.c
+++ b/init/do_mounts.c
@@ -500,7 +500,7 @@ void __init change_floppy(char *fmt, ...)
 		ksys_ioctl(fd, TCGETS, (long)&termios);
 		termios.c_lflag &= ~ICANON;
 		ksys_ioctl(fd, TCSETSF, (long)&termios);
-		sys_read(fd, &c, 1);
+		ksys_read(fd, &c, 1);
 		termios.c_lflag |= ICANON;
 		ksys_ioctl(fd, TCSETSF, (long)&termios);
 		ksys_close(fd);