ipmi:msghandler:Change seq_lock to a mutex

Dan Carpenter got a Smatch warning:

	drivers/char/ipmi/ipmi_msghandler.c:5265 ipmi_free_recv_msg()
	warn: sleeping in atomic context

due to the recent rework of the IPMI driver's locking.  I didn't realize
vfree could block.  But there is an easy solution to this, now that
almost everything in the message handler runs in thread context.

I wanted to spend the time earlier to see if seq_lock could be converted
from a spinlock to a mutex, but I wanted the previous changes to go in
and soak before I did that.  So I went ahead and did the analysis and
converting should work.  And solve this problem.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/r/202503240244.LR7pOwyr-lkp@intel.com/


Fixes: 3be997d5 ("ipmi:msghandler: Remove srcu from the ipmi user structure")
Cc: <stable@vger.kernel.org> # 6.16
Signed-off-by: Corey Minyard <corey@minyard.net>