libzfs_sendrecv: Factor out lzc_flags_from_resume_nvl

Improve the readability of zfs_send_resume_impl by moving resume nvl
decoding into a separate helper function.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <freqlabs@FreeBSD.org>
Closes #12967