ksmbd: discard write access to the directory open

In the Linux kernel, the following vulnerability has been resolved:

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