locks: create a new notifier chain for lease attempts

With the new file caching infrastructure in nfsd, we can end up holding
files open for an indefinite period of time, even when they are still
idle. This may prevent the kernel from handing out leases on the file,
which is something we don't want to block.

Fix this by running a SRCU notifier call chain whenever on any
lease attempt. nfsd can then purge the cache for that inode before
returning.

Since SRCU is only conditionally compiled in, we must only define the
new chain if it's enabled, and users of the chain must ensure that
SRCU is enabled.

Signed-off-by: Jeff Layton <jeff.layton@primarydata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>