arch/frv/kernel/traps.c: using vsnprintf() instead of vsprintf()

Since die_if_kernel() is an extern common used function, better always
check the buffer length to avoid memory overflow by a long 'str'.

Signed-off-by: Chen Gang <gang.chen@asianux.com>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>