Suppress Clang Static Analyzer warning in bpobj_enqueue()

scan-build does not do cross translation unit analysis to realize that
dmu_buf_hold() will always set bpo->bpo_cached_dbuf to a non-NULL
pointer, so we add an assertion to make it realize this.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14575