scsi: fusion: fix string overflow warning

gcc points out a theorerical string overflow:

drivers/message/fusion/mptbase.c: In function 'mpt_detach':
drivers/message/fusion/mptbase.c:2103:17: error: '%s' directive writing up to 31 bytes into a region of size 28 [-Werror=format-overflow=]
sprintf(pname, MPT_PROCFS_MPTBASEDIR "/%s/summary", ioc->name);
               ^~~~~
drivers/message/fusion/mptbase.c:2103:2: note: 'sprintf' output between 13 and 44 bytes into a destination of size 32

We can simply double the size of the local buffer here to be on the
safe side, and using snprintf() instead of sprintf() protects us
if ioc->name was not terminated properly.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>