snapuserd: I/O request on sectors not mapping to any COW op

If there is an I/O request which doesn't map to any
COW Operation in the vector mapping, then that request
is an EOF request and should not be discarded with an
I/O error.

Bug: 198787355
Test: Incremental OTA
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I241ee11d7cf540854aa4a232bc1b18da43d41e05