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 openzfs#16643

module/os/freebsd/zfs/zfs_vnops_os.c

@@ -452,8 +452,10 @@ mappedread_sf(znode_t *zp, int nbytes, zfs_uio_t *uio)
 				if (!vm_page_wired(pp) && pp->valid == 0 &&
 				    vm_page_busy_tryupgrade(pp))
 					vm_page_free(pp);
-				else
+				else {
+					vm_page_deactivate_noreuse(pp);
 					vm_page_sunbusy(pp);
+				}
 				zfs_vmobject_wunlock(obj);
 			}
 		} else {