Cleanup dump_bookmarks()

Assertions are meant to check assumptions, but the way that this
assertion is written does not check an assumption, since it is provably
always true. Removing the assertion will cause a compiler warning (made
into an error by -Werror) about printing up to 512 bytes to a 256-byte
buffer, so instead, we change the assertion to verify the assumption
that we never do a snprintf() that is truncated to avoid overrunning the
256-byte buffer.

This was caught by an audit of the codebase to look for misuse of
`snprintf()` after CodeQL reported that we had misused `snprintf()`. An
explanation of how snprintf() can be misused is here:

https://www.redhat.com/en/blog/trouble-snprintf

This particular instance did not misuse `snprintf()`, but it was caught
by the audit anyway.

Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Richard Yao <[email protected]>
Closes openzfs#14098

cmd/zdb/zdb.c

@@ -2858,9 +2858,11 @@ dump_bookmarks(objset_t *os, int verbosity)
 	    zap_cursor_advance(&zc)) {
 		char osname[ZFS_MAX_DATASET_NAME_LEN];
 		char buf[ZFS_MAX_DATASET_NAME_LEN];
+		int len;
 		dmu_objset_name(os, osname);
-		VERIFY3S(0, <=, snprintf(buf, sizeof (buf), "%s#%s", osname,
-		    attr.za_name));
+		len = snprintf(buf, sizeof (buf), "%s#%s", osname,
+		    attr.za_name);
+		VERIFY3S(len, <, ZFS_MAX_DATASET_NAME_LEN);
 		(void) dump_bookmark(dp, buf, verbosity >= 5, verbosity >= 6);
 	}
 	zap_cursor_fini(&zc);