[XFS] Prevent buffer overrun in cmn_err().

The message buffer used by cmn_err() is only 256 bytes and some CXFS
messages were exceeding this length. Since we were using vsprintf() and
not checking for buffer overruns we were clobbering memory beyond the
buffer. The size of the buffer has been increased to 1024 bytes so we can
capture these larger messages and we are now using vsnprintf() to prevent
overrunning the buffer size.

SGI-PV: 958599
SGI-Modid: xfs-linux-melb:xfs-kern:27561a

Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>
Signed-off-by: Geoffrey Wehrman <gwehrman@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>