ksmbd: fix use-after-free of fp->owner.name in durable handle owner check

ksmbd: fix use-after-free of fp->owner.name in durable handle owner check

Two concurrent SMB2 durable reconnects (DH2C/DHnC) on the same
persistent_id race the fp->owner.name compare-read in
ksmbd_vfs_compare_durable_owner() against the kfree() in
ksmbd_reopen_durable_fd()'s reopen-success path. fp->owner.name is a
standalone kstrdup() buffer whose lifetime is independent of the fp
refcount, and the two sites share no lock: the compare reads the buffer
while the reopen frees it, so the strcmp() can dereference freed memory.

Commit 7ce4fc40018d ("ksmbd: fix durable reconnect double-bind race in
ksmbd_reopen_durable_fd") made the fp->conn claim atomic under
global_ft.lock (closing the owner.name double-free and the ksmbd_file
write-UAF), but the compare-read versus reopen-free pair was left
unserialized.

  BUG: KASAN: slab-use-after-free in strcmp+0x2c/0x80
  Read of size 1 by task kworker
    strcmp
    ksmbd_vfs_compare_durable_owner
    smb2_check_durable_oplock
    smb2_open
  Freed by task kworker:
    kfree
    ksmbd_reopen_durable_fd
    smb2_open
  Allocated by task kworker:
    kstrdup
    session_fd_check
    smb2_session_logoff
  The buggy address belongs to the cache kmalloc-8

Serialize both sides of the race with fp->f_lock.  The global durable
file-table lock still protects the durable reconnect claim, but
fp->owner.name is per-open state and does not need to block unrelated
durable table lookups or reconnects.  The teardown is left at its
existing location after the reopen-success point so that an __open_id()
rollback still retains owner.name for a later legitimate reconnect to
verify.

Fixes: 49110a8ce654 ("ksmbd: validate owner of durable handle on reconnect")
Assisted-by: Henry (Claude):claude-opus-4
Signed-off-by: Gil Portnoy <dddhkts1@gmail.com>
Co-developed-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>