checkpatch: volatile with a comment or sig_atomic_t is okay

This assumes that the comment gives some justification;
"volatile sig_atomic_t" is also self-explanatory and usually
correct.

Discussed in:
'[Qemu-devel] [PATCH] dump-guest-memory.py: fix "You can't do that without a process to debug"'

Suggested-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20171215181810.4122-1-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
