cifs: fix session state transition to avoid use-after-free issue

We switch session state to SES_EXITING without cifs_tcp_ses_lock now,
it may lead to potential use-after-free issue.

Consider the following execution processes:

Thread 1:
__cifs_put_smb_ses()
    spin_lock(&cifs_tcp_ses_lock)
    if (--ses->ses_count > 0)
        spin_unlock(&cifs_tcp_ses_lock)
        return
    spin_unlock(&cifs_tcp_ses_lock)
        ---> **GAP**
    spin_lock(&ses->ses_lock)
    if (ses->ses_status == SES_GOOD)
        ses->ses_status = SES_EXITING
    spin_unlock(&ses->ses_lock)

Thread 2:
cifs_find_smb_ses()
    spin_lock(&cifs_tcp_ses_lock)
    list_for_each_entry(ses, ...)
        spin_lock(&ses->ses_lock)
        if (ses->ses_status == SES_EXITING)
            spin_unlock(&ses->ses_lock)
            continue
        ...
        spin_unlock(&ses->ses_lock)
    if (ret)
        cifs_smb_ses_inc_refcount(ret)
    spin_unlock(&cifs_tcp_ses_lock)

If thread 1 is preempted in the gap and thread 2 start executing, thread 2
will get the session, and soon thread 1 will switch the session state to
SES_EXITING and start releasing it, even though thread 1 had increased the
session's refcount and still uses it.

So switch session state under cifs_tcp_ses_lock to eliminate this gap.

Signed-off-by: Winston Wen <wentao@uniontech.com>
Signed-off-by: Steve French <stfrench@microsoft.com>