nfsd: make sure exp active before svc_export_show

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

nfsd: make sure exp active before svc_export_show

The function `e_show` was called with protection from RCU. This only

ensures that `exp` will not be freed. Therefore, the reference count for

`exp` can drop to zero, which will trigger a refcount use-after-free

warning when `exp_get` is called. To resolve this issue, use

`cache_get_rcu` to ensure that `exp` remains active.

------------[ cut here ]------------

refcount_t: addition on 0; use-after-free.

WARNING: CPU: 3 PID: 819 at lib/refcount.c:25

refcount_warn_saturate+0xb1/0x120

CPU: 3 UID: 0 PID: 819 Comm: cat Not tainted 6.12.0-rc3+ #1

Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS

1.16.1-2.fc37 04/01/2014

RIP: 0010:refcount_warn_saturate+0xb1/0x120

...

Call Trace:

<TASK>

e_show+0x20b/0x230 [nfsd]

seq_read_iter+0x589/0x770

seq_read+0x1e5/0x270

vfs_read+0x125/0x530

ksys_read+0xc1/0x160

do_syscall_64+0x5f/0x170

entry_SYSCALL_64_after_hwframe+0x76/0x7e