Allow multiple values per directory entry

Allow multiple values per directory entry

When using zfs to back a Lustre filesystem it's advantageous to
to store a fid with the object id in the directory zap.  The only
technical impediment to doing this is that the zpl code expects
a single value in the zap per directory entry.

This change relaxes that requirement such that multiple entries
are allowed provided the first one is the object id.  The zpl
code will just ignore additional entries.  This allows the ZoL
count to mount datasets which are being used as Lustre server
backends.

Once the upstream feature flags support is merged in this change
should be updated to a read-only feature.  Until this occurs
other zfs implementations will not be able to read the zfs
filesystems created by Lustre.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>