ksmbd: Fix UAF in __close_file_table_ids

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

ksmbd: Fix UAF in __close_file_table_ids

A use-after-free is possible if one thread destroys the file

via __ksmbd_close_fd while another thread holds a reference to

it. The existing checks on fp->refcount are not sufficient to

prevent this.

The fix takes ft->lock around the section which removes the

file from the file table. This prevents two threads acquiring the

same file pointer via __close_file_table_ids, as well as the other

functions which retrieve a file from the IDR and which already use

this same lock.