cifs: fix DFS traversal oops without CONFIG_CIFS_DFS_UPCALL

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

cifs: fix DFS traversal oops without CONFIG_CIFS_DFS_UPCALL

When compiled with CONFIG_CIFS_DFS_UPCALL disabled, cifs_dfs_d_automount

is NULL. cifs.ko logic for mapping CIFS_FATTR_DFS_REFERRAL attributes to

S_AUTOMOUNT and corresponding dentry flags is retained regardless of

CONFIG_CIFS_DFS_UPCALL, leading to a NULL pointer dereference in

VFS follow_automount() when traversing a DFS referral link:

BUG: kernel NULL pointer dereference, address: 0000000000000000

...

Call Trace:

<TASK>

__traverse_mounts+0xb5/0x220

? cifs_revalidate_mapping+0x65/0xc0 [cifs]

step_into+0x195/0x610

? lookup_fast+0xe2/0xf0

path_lookupat+0x64/0x140

filename_lookup+0xc2/0x140

? __create_object+0x299/0x380

? kmem_cache_alloc+0x119/0x220

? user_path_at_empty+0x31/0x50

user_path_at_empty+0x31/0x50

__x64_sys_chdir+0x2a/0xd0

? exit_to_user_mode_prepare+0xca/0x100

do_syscall_64+0x42/0x90

entry_SYSCALL_64_after_hwframe+0x72/0xdc

This fix adds an inline cifs_dfs_d_automount() {return -EREMOTE} handler

when CONFIG_CIFS_DFS_UPCALL is disabled. An alternative would be to

avoid flagging S_AUTOMOUNT, etc. without CONFIG_CIFS_DFS_UPCALL. This

approach was chosen as it provides more control over the error path.