nfs4: Fix mismatching allocation and deallocation

Fixes CID #1108170.

Signed-off-by: Pawel Dziepak <pdziepak@quarnos.org>
nfs4: Fix CID 1108216: Non-array delete used on array
nfs4: Fix CID 1108337: Use AutoDeleter to fix leak
nfs4: Fix CID 1108338: Fix memory leak
nfs4: FileSystem::~FileSystem() make sure fServer is valid