cifs: Release folio lock on fscache read hit.

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

cifs: Release folio lock on fscache read hit.

Under the current code, when cifs_readpage_worker is called, the call

contract is that the callee should unlock the page. This is documented

in the read_folio section of Documentation/filesystems/vfs.rst as:

> The filesystem should unlock the folio once the read has completed,

> whether it was successful or not.

Without this change, when fscache is in use and cache hit occurs during

a read, the page lock is leaked, producing the following stack on

subsequent reads (via mmap) to the page:

$ cat /proc/3890/task/12864/stack

[<0>] folio_wait_bit_common+0x124/0x350

[<0>] filemap_read_folio+0xad/0xf0

[<0>] filemap_fault+0x8b1/0xab0

[<0>] __do_fault+0x39/0x150

[<0>] do_fault+0x25c/0x3e0

[<0>] __handle_mm_fault+0x6ca/0xc70

[<0>] handle_mm_fault+0xe9/0x350

[<0>] do_user_addr_fault+0x225/0x6c0

[<0>] exc_page_fault+0x84/0x1b0

[<0>] asm_exc_page_fault+0x27/0x30

This requires a reboot to resolve; it is a deadlock.

Note however that the call to cifs_readpage_from_fscache does mark the

page clean, but does not free the folio lock. This happens in

__cifs_readpage_from_fscache on success. Releasing the lock at that

point however is not appropriate as cifs_readahead also calls

cifs_readpage_from_fscache and *does* unconditionally release the lock

after its return. This change therefore effectively makes

cifs_readpage_worker work like cifs_readahead.