nfsd: accept config to set number of threads

[Thomas: tweak commit title and author.]

Signed-off-by: Andreas Ehmanns <universeII@gmx.de>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>