sunrpc: avoid -Wformat-security warning

Using a non-constant string as an sprintf-style is potentially dangerous:

net/sunrpc/svc.c: In function 'param_get_pool_mode':
net/sunrpc/svc.c:164:32: error: format not a string literal and no format arguments [-Werror=format-security]

Use a literal "%s" format instead.

Fixes: 5f71f3c3 ("sunrpc: refactor pool_mode setting code")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>