Fix noop receive of raw send stream

Currently, the noop receive code fails to work with raw send streams
and resuming send streams. This happens because zfs_receive_impl()
reads the DRR_BEGIN payload without reading the payload itself.
Normally, the kernel expects to read this itself, but in this case
the recv_skip() code runs instead and it is not prepared to handle
the stream being left at any place other than the beginning of a
record.

This patch resolves this issue by manually reading the DRR_BEGIN
payload in the dry-run case. This patch also includes a number of
small fixups in this code path.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Paul Dagnelie <pcd@delphix.com>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #9221
Closes #9173

tests/zfs-tests/tests/functional/cli_root/zfs_receive/zfs_receive_raw.ksh

@@ -36,6 +36,7 @@
 # 9. Verify the key is unavailable
 # 10. Attempt to load the key and mount the dataset
 # 11. Verify the checksum of the file is the same as the original
+# 12. Verify 'zfs receive -n' works with the raw stream
 #
 
 verify_runnable "both"
@@ -88,4 +89,6 @@ typeset cksum2=$(md5digest /$TESTPOOL/$TESTFS1/c1/$TESTFILE0)
 [[ "$cksum2" == "$checksum" ]] || \
 	log_fail "Checksums differ ($cksum2 != $checksum)"
 
+log_must eval "zfs send -w $snap | zfs receive -n $TESTPOOL/$TESTFS3"
+
 log_pass "ZFS can receive streams from raw sends"