NFSD: refactoring courtesy_client_reaper to a generic low memory shrinker

[ Upstream commit a1049eb4

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Refactoring courtesy_client_reaper to generic low memory
shrinker so it can be used for other purposes.

Signed-off-by: Dai Ngo <dai.ngo@oracle.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>