mm/cma_debug.c: align the name buffer length as struct cma

Avoids truncating the debugfs output to 16 chars.  Potentially alters
the userspace output, but this is a debugfs interface and there are no
stability guarantees.

Link: https://lkml.kernel.org/r/20220719091554.27864-1-quic_yingangl@quicinc.com
Signed-off-by: Kassey Li <quic_yingangl@quicinc.com>
Cc: Sasha Levin <sashal@kernel.org>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>