Use zap_count instead of cached z_size for unlink

Use zap_count instead of cached z_size for unlink

As a performance optimization Lustre does not strictly update
the SA_ZPL_SIZE when adding/removing from non-directory entries.
This results in entries which cannot be removed through the ZPL
layer even though the ZAP is empty and safe to remove.

Resolve this issue by checking the zap_count() directly instead
on relying on the cached SA_ZPL_SIZE.  Micro-benchmarks show no
significant performance impact due to the additional overhead
of using zap_count().

Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7019