parasite: remove restriction to a number of iovec-s

vmsplice can't splice more than UIO_MAXIOV, but we can
call it a few times from a parasite.

v2: s/nr/nr_segs/

Acked-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>