fs/nfsd: change type of max_delegations, nfsd_drc_max_mem and nfsd_drc_mem_used

The three variables are calculated from nr_free_buffer_pages so change
their types to unsigned long in case of overflow.

Signed-off-by: Zhang Yanfei <zhangyanfei@cn.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>