add get_name implementation for exports. (#16833)

This fixes a serious performance problem with NFS handling of large
directories, as the new get_name code is much more efficient than the
default zfs_readdir. This is actually part of
20232ecfaa34177bef6c08f2f1a55b8c8bd20da4 in 2.3. But I've taken only
the minimum code to implement get_name, and not the rest of the long
name changes.

Signed-off-by: Charles Hedrick <hedrick@rutgers.edu>
Co-authored-by: Charles L. Hedrick <hedrick@ncommunis.cs.rutgers.edu>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>