tty_buffer: Fix distinct type warning

CC      drivers/char/tty_buffer.o
drivers/char/tty_buffer.c: In function ‘tty_insert_flip_string_fixed_flag’:
drivers/char/tty_buffer.c:251: warning: comparison of distinct pointer types lacks a cast
drivers/char/tty_buffer.c: In function ‘tty_insert_flip_string_flags’:
drivers/char/tty_buffer.c:288: warning: comparison of distinct pointer types lacks a cast

Fix it by replacing min() with min_t() in tty_insert_flip_string_flags and
					  tty_insert_flip_string_fixed_flag().

Signed-off-by: Fang Wenqi <antonf@turbolinux.com.cn>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>