9p/trans_fd: mark concurrent read and writes to p9_conn->err

Writes for the error value of a connection are spinlock-protected inside
p9_conn_cancel, but lockless reads are present elsewhere to avoid
performing unnecessary work after an error has been met.

Mark the write and lockless reads to make KCSAN happy. Mark the write as
exclusive following the recommendation in "Lock-Protected Writes with
Lockless Reads" in tools/memory-model/Documentation/access-marking.txt
while we are at it.

Mark p9_fd_request and p9_conn_cancel m->err reads despite the fact that
they do not race with concurrent writes for stylistic reasons.

Reported-by:  <syzbot+d69a7cc8c683c2cb7506@syzkaller.appspotmail.com>
Reported-by:  <syzbot+483d6c9b9231ea7e1851@syzkaller.appspotmail.com>
Signed-off-by: Ignacio Encinas <ignacio@iencinas.com>
Message-ID: <20250318-p9_conn_err_benign_data_race-v3-1-290bb18335cc@iencinas.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>