md/raid5: Increase r5conf.cache_name size

For compiling with W=1, the following warning can be seen:

drivers/md/raid5.c: In function ‘setup_conf’:
drivers/md/raid5.c:2423:12: error: ‘%s’ directive output may be truncated writing up to 31 bytes into a region of size between 16 and 26 [-Werror=format-truncation=]
    "raid%d-%s", conf->level, mdname(conf->mddev));
            ^~
drivers/md/raid5.c:2422:3: note: ‘snprintf’ output between 7 and 48 bytes into a destination of size 32
   snprintf(conf->cache_name[0], namelen,
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    "raid%d-%s", conf->level, mdname(conf->mddev));
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

Increase the array size to avoid this warning.

Signed-off-by: John Garry <john.g.garry@oracle.com>
Link: https://lore.kernel.org/r/20241112161019.4154616-2-john.g.garry@oracle.com


Signed-off-by: Song Liu <song@kernel.org>