NFS: take a delegation reference in nfs4_get_valid_delegation

NFS: take a delegation reference in nfs4_get_valid_delegation

Currently most work on struct nfs_delegation happens directly under RCU
protection.  This is generally fine, despite that long RCU sections are
not good for performance.  But for operations later taking a reference
to the delegation to perform blocking work, refcount_inc is used, which
can be racy against dropping the last reference and thus lead to use
after frees in extremely rare cases.

Fix this by taking a reference in nfs4_get_valid_delegation using
refcount_inc_not_zero so that the callers have a stabilized reference
they can work with and can be moved outside the RCU critical section.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>