Fix ARC target collapse when zfs_arc_meta_limit_percent=100

Reclaim metadata when arc_available_memory < 0 even if
meta_used is not bigger than arc_meta_limit.

As described in https://github.com/openzfs/zfs/issues/14054 if
zfs_arc_meta_limit_percent=100 then ARC target can collapse to
arc_min due to arc_purge not freeing any metadata.

This patch lets arc_prune to do its work when arc_available_memory
is negative even if meta_used is not bigger than arc_meta_limit,
avoiding ARC target collapse.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Gionatan Danti <g.danti@assyoma.it>
Closes #14054
Closes #14093