sendfile: Ensure that sfio->npages is initialized

[markj]

Summary

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.

Test Plan

syzkaller triggered the bug and KASAN detected it because the uninitialized
value caused the loop in sendfile_iodone() to run past the end of the buffer.