nfsd: don't free files unconditionally in __nfsd_file_cache_purge

nfsd: don't free files unconditionally in __nfsd_file_cache_purge

[ Upstream commit 41c43bade5acdacecb2d8aa9fa58ed6b3f975e26 ]

nfsd_file_cache_purge is called when the server is shutting down, in
which case, tearing things down is generally fine, but it also gets
called when the exports cache is flushed.

Instead of walking the cache and freeing everything unconditionally,
handle it the same as when we have a notification of conflicting access.

Fixes: d909bd75e7ca ("nfsd: rework refcounting in filecache")
Reported-by: Ruben Vestergaard <rubenv@drcmr.dk>
Reported-by: Torkil Svensgaard <torkil@drcmr.dk>
Reported-by: Shachar Kagan <skagan@nvidia.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Tested-by: Shachar Kagan <skagan@nvidia.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>