nfsd: Get rid of useless "pool" field in nfs_fha_new.c

Since svcpool_create() is now called from an initialization function,
the pool field of fha_params is always non-NULL, so just get
rid of it and the useless check for it being NULL.

(cherry picked from commit b99c419587db2ac75b929637f61ec3b845d7b5f6)