smb: client: add mid_counter_lock to protect the mid counter counter

This is step 2/4 of a patch series to fix mid_q_entry memory leaks
caused by race conditions in callback execution.

Add a dedicated mid_counter_lock to protect current_mid counter,
separating it from mid_queue_lock which protects pending_mid_q
operations. This reduces lock contention and prepares for finer-
grained locking in subsequent patches.

Changes:
- Add TCP_Server_Info->mid_counter_lock spinlock
- Rename CurrentMid to current_mid for consistency
- Use mid_counter_lock to protect current_mid access
- Update locking documentation in cifsglob.h

This separation allows mid allocation to proceed without blocking
queue operations, improving performance under heavy load.

Signed-off-by: Wang Zhaolong <wangzhaolong@huaweicloud.com>
Acked-by: Enzo Matsumiya <ematsumiya@suse.de>
Signed-off-by: Steve French <stfrench@microsoft.com>