Remove dead code and cleanup scoping in dmu_send.c

Remove dead code and cleanup scoping in dmu_send.c

This addresses a number of problems with dmu_send.c:

* bp_span is unused which makes clang complain
* dump_write conflicts with FreeBSD's existing core dump code
* range_alloc is private to the file and not declared in any headers
  causing clang to complain

Reviewed-by: Matt Ahrens <matt@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #9432