net/9p: use a dedicated spinlock for trans_fd

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

net/9p: use a dedicated spinlock for trans_fd

Shamelessly copying the explanation from Tetsuo Handa's suggested

patch[1] (slightly reworded):

syzbot is reporting inconsistent lock state in p9_req_put()[2],

for p9_tag_remove() from p9_req_put() from IRQ context is using

spin_lock_irqsave() on "struct p9_client"->lock but trans_fd

(not from IRQ context) is using spin_lock().

Since the locks actually protect different things in client.c and in

trans_fd.c, just replace trans_fd.c's lock by a new one specific to the

transport (client.c's protect the idr for fid/tag allocations,

while trans_fd.c's protects its own req list and request status field

that acts as the transport's state machine)