ksmbd: discard write access to the directory open

may_open() does not allow a directory to be opened with the write access.
However, some writing flags set by client result in adding write access
on server, making ksmbd incompatible with FUSE file system. Simply, let's
discard the write access when opening a directory.

list_add corruption. next is NULL.
------------[ cut here ]------------
kernel BUG at lib/list_debug.c:26!
pc : __list_add_valid+0x88/0xbc
lr : __list_add_valid+0x88/0xbc
Call trace:
__list_add_valid+0x88/0xbc
fuse_finish_open+0x11c/0x170
fuse_open_common+0x284/0x5e8
fuse_dir_open+0x14/0x24
do_dentry_open+0x2a4/0x4e0
dentry_open+0x50/0x80
smb2_open+0xbe4/0x15a4
handle_ksmbd_work+0x478/0x5ec
process_one_work+0x1b4/0x448
worker_thread+0x25c/0x430
kthread+0x104/0x1d4
ret_from_fork+0x10/0x20

Cc: stable@vger.kernel.org
Signed-off-by: Yoonho Shin <yoonho.shin@samsung.com>
Signed-off-by: Hobin Woo <hobin.woo@samsung.com>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>