[PATCH] kNFSd: Protect reference to exp across calls to nfsd_cross_mnt

[PATCH] kNFSd: Protect reference to exp across calls to nfsd_cross_mnt

From: NeilBrown <neilb@cse.unsw.edu.au>

nfsd_cross_mnt can release the reference to the passed svc_export structure
when it returns a different svc_export structure.  So we need to make sure we
have a counted reference before, and drop the reference afterwards.