ksmbd: reset rcount per connection in ksmbd_conn_wait_idle_sess_id()

rcount is intended to be connection-specific: 2 for curr_conn, 1 for
every other connection sharing the same session. However, it is
initialised only once before the hash iteration and is never reset.
After the loop visits curr_conn, later sibling connections are also
checked against rcount == 2, so a sibling with req_running == 1 is
incorrectly treated as idle. This makes the outcome depend on the
hash iteration order: whether a given sibling is checked against the
loose (< 2) or the strict (< 1) threshold is decided by whether it
happens to be visited before or after curr_conn.

The function's contract is "wait until every connection sharing this
session is idle" so that destroy_previous_session() can safely tear
the session down. The latched rcount violates that contract and
reopens the teardown race window the wait logic was meant to close:
destroy_previous_session() may proceed before sibling channels have
actually quiesced, overlapping session teardown with in-flight work
on those connections.

Recompute rcount inside the loop so each connection is compared
against its own threshold regardless of iteration order.

This is a code-inspection fix for an iteration-order-dependent logic
error; a targeted reproducer would require SMB3 multichannel with
in-flight work on a sibling channel landing after curr_conn in hash
order, which is not something that can be triggered reliably.

Fixes: 76e98a158b20 ("ksmbd: fix race condition between destroy_previous_session() and smb2 operations()")
Cc: stable@vger.kernel.org
Signed-off-by: DaeMyung Kang <charsyam@gmail.com>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>