mqueue: correct the type of ro to int

mqueue: correct the type of ro to int

The ro variable, being of type bool, caused the -EROFS return value from
mnt_want_write() to be implicitly converted to 1. This prevented the file
from being correctly acquired, thus triggering the issue reported by
syzbot [1].

Changing the type of ro to int allows the system to correctly identify
the reason for the file open failure.

[1]
KASAN: null-ptr-deref in range [0x0000000000000040-0x0000000000000047]
Call Trace:
 do_mq_open+0x5a0/0x770 ipc/mqueue.c:932
 __do_sys_mq_open ipc/mqueue.c:945 [inline]
 __se_sys_mq_open ipc/mqueue.c:938 [inline]
 __x64_sys_mq_open+0x16a/0x1c0 ipc/mqueue.c:938

Fixes: f2573685bd0c ("ipc: convert do_mq_open() to FD_ADD()")
Reported-by: syzbot+40f42779048f7476e2e0@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=40f42779048f7476e2e0
Tested-by: syzbot+40f42779048f7476e2e0@syzkaller.appspotmail.com
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
Link: https://patch.msgid.link/tencent_369728EA76ED36CD98793A6D942C956C4C0A@qq.com
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>