NFS: Add a helper nfs_client_for_each_server()

[ Upstream commit 3c9e502b ]

Add a helper nfs_client_for_each_server() to iterate through all the
filesystems that are attached to a struct nfs_client, and apply
a function to all the active ones.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Stable-dep-of: ed1cc05a ("NFSv4: Fix a nfs4_state_manager() race")
Signed-off-by: Sasha Levin <sashal@kernel.org>