kernel/gcov/fs.c: use pr_warn()

pr_warning() is deprecated in favor of pr_warn()

Cc: Andy Gospodarek <agospoda@redhat.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Frantisek Hrbata <fhrbata@redhat.com>
Cc: Jan Stancek <jstancek@redhat.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Peter Oberparleiter <peter.oberparleiter@de.ibm.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>