Fix HIGHMEM/kmap API violation in zfs_uiomove_bvec_impl()

Fix another instance where ZFS assumes multiple pages can be
mapped at once via zfs_kmap_local(), resulting in crashes and
potential memory corruption on HIGHMEM-enabled (typically 32-bit)
systems.

Reviewed-by: RageLtMan <rageltman@sempervictus>
Reviewed-by: Rob Norris <robn@despairlabs.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: bspengler-oss <94915855+bspengler-oss@users.noreply.github.com>
Closes #15668
Closes #18030