NFSD: Fix problem of COMMIT and NFS4ERR_DELAY in infinite loop

NFSD: Fix problem of COMMIT and NFS4ERR_DELAY in infinite loop

The following request sequence to the same file causes the NFS client and
server getting into an infinite loop with COMMIT and NFS4ERR_DELAY:

OPEN
REMOVE
WRITE
COMMIT

Problem reported by recall11, recall12, recall14, recall20, recall22,
recall40, recall42, recall48, recall50 of nfstest suite.

This patch restores the handling of race condition in nfsd_file_do_acquire
with unlink to that prior of the regression.

Fixes: ac3a2585f018 ("nfsd: rework refcounting in filecache")
Signed-off-by: Dai Ngo <dai.ngo@oracle.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>