sendfile: Ensure that sfio->npages is initialized

We initialize sfio->npages only when some I/O is required to satisfy the
request. However, sendfile_iodone() contains an INVARIANTS-only check
that references sfio->npages, and this check is executed even if no I/O
is performed, so the check may use an uninitialized value.

Fix the problem by initializing sfio->npages earlier. Note that
sendfile_swapin() always initializes the page array. In some rare cases
we need to trim the page array so ensure that sfio->npages gets updated
accordingly.

Reported by: syzkaller (with KASAN)
Reviewed by: kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D27726