Fix a potential page leak in mappedread_sf()

mappedread_sf() may allocate pages; if it fails to populate a page
can't free it, it needs to ensure that it's placed into a page queue,
otherwise it can't be reclaimed until the vnode is destroyed.

I think this is quite unlikely to happen in practice, it was noticed by
code inspection.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Signed-off-by: Mark Johnston <markj@FreeBSD.org>
Closes #16643