target-ppc: Document TOCTTOU in hugepage support

The code to find the minimum page size is is vulnerable to TOCTTOU.
Added in commit 2d103aa "target-ppc: fix hugepage support when using
memory-backend-file" (v2.4.0).  Since I can't fix it myself right now,
add a FIXME comment.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1458066895-20632-2-git-send-email-armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
