FreeBSD: zfs_getpages: Don't zero freshly allocated pages

Initially, zfs_getpages() is provided with an array of busy pages by
the vnode pager. It then tries to acquire the range lock, but if there
is a concurrent zfs_write() running and fails to acquire that range
lock, it "unbusies" the pages to avoid a deadlock with zfs_write().
After that, it grabs the pages again and retries to acquire the range
lock, and so on.

Once it got the range lock, it filters out valid pages, then copy DMU
data to the remaining invalid pages.

The problem is that freshly allocated zero'd pages it grabbed itself are
marked as valid. Therefore they are skipped by the second part of the
function and DMU data is never copied to these pages. This causes mapped
pages to contain zeros instead of the expected file content.

This was discovered while working on RabbitMQ on FreeBSD. I could
reproduce the problem easily with the following commands:

git clone https://github.com/rabbitmq/rabbitmq-server.git
cd rabbitmq-server/deps/rabbit

gmake distclean-ct RABBITMQ_METADATA_STORE=mnesia \
  ct-amqp_client t=cluster_size_3:leader_transfer_stream_send

The testsuite fails because there is a sendfile(2) that can happen
concurrently to a write(2) on the same file. This leads to sendfile(2)
or read(2) (after the sendfile) sending/returning data with zeros, which
causes a function to crash.

The patch consists of not setting the VM_ALLOC_ZERO flag when
zfs_getpages() grabs pages again. Then, the last page is zero'd if it
is invalid, in case it would be partially filled with the end of the
file content. Other pages are either valid (and will be skipped) or they
will be entirely overwritten by the file content.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Mark Johnston <markj@FreeBSD.org>
Signed-off-by: Jean-Sébastien Pédron <dumbbell@FreeBSD.org>
Closes #17851