signal/sh: Use force_sig(SIGKILL) instead of do_group_exit(SIGKILL)

Today the sh code allocates memory the first time a process uses
the fpu.  If that memory allocation fails, kill the affected task
with force_sig(SIGKILL) rather than do_group_exit(SIGKILL).

Calling do_group_exit from an exception handler can potentially lead
to dead locks as do_group_exit is not designed to be called from
interrupt context.  Instead use force_sig(SIGKILL) to kill the
userspace process.  Sending signals in general and force_sig in
particular has been tested from interrupt context so there should be
no problems.

Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc: Rich Felker <dalias@libc.org>
Cc: linux-sh@vger.kernel.org
Fixes: 0ea820cf ("sh: Move over to dynamically allocated FPU context.")
Link: https://lkml.kernel.org/r/20211020174406.17889-6-ebiederm@xmission.com


Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>