Correct assert added to dump program.

The dump program was exiting with the message:

Assertion failed: (spcl.c_count + blks < TP_NINDIR), function appendextdata, file /usr/src/sbin/dump/traverse.c, line 759.

The problem arose when dumping external attributes.

This assertion was added in this commit with no review by someone
with expertise in the dump program:

commit 2d518c6518cdb256ff6f2c463e6b115d89c104c3
Author:     Warner Losh <imp@FreeBSD.org>
AuthorDate: Mon Jun 11 19:32:36 2018 +0000
Commit:     Warner Losh <imp@FreeBSD.org>
CommitDate: Mon Jun 11 19:32:36 2018 +0000

Add asserts to prevent overflows of c_addr.

It is clearly wrong as the statement immediately above it in the
code which is deciding if the data will fit is:

if (spcl.c_count + blks > TP_NINDIR)

		return (0);

As is pointed out in the bug report, the assert should be:

(spcl.c_count + blks <= TP_NINDIR)

This commit corrects the assert. I am sorry that it took so long to
be brought to my attention and get fixed.

Reported by: Hampton Finger
PR: 244470
MFC after: 3 days
Sponsored by: Netflix