Don't dirty bpobj if it has no entries

Don't dirty bpobj if it has no entries

In certain cases (dsl_scan_sync() is one), we may end up calling
bpobj_iterate() on an empty bpobj. Even though we don't end up
modifying the bpobj it still gets dirtied, causing unneeded writes
to the pool.

This patch adds an early bail from bpobj_iterate_impl() if bpobj
is empty to prevent unneeded writes.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: Alek Pinchuk <apinchuk@datto.com>
Closes #6164