stand: zfs: handle holes at the tail end correctly

This mirrors dmu_read_impl(), zeroing out the tail end of the buffer and
clipping the read to what's contained by the block that exists.

This fixes an issue that arose during the 13.1 release process; in
13.1-RC1 and later, setting up GELI+ZFS will result in a failure to
boot.  The culprit is this, which causes us to fail to load geom_eli.ko
as there's a residual portion after the single datablk that should be
zeroed out.

PR:		263407
Reviewed by:	tsoome
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D35019

stand/libsa/zfs/zfsimpl.c

@@ -2349,6 +2349,19 @@ dnode_read(const spa_t *spa, const dnode_phys_t *dnode, off_t offset,
 		return (EIO);
 	}
 
+	/*
+	 * Handle odd block sizes, mirrors dmu_read_impl().  Data can't exist
+	 * past the first block, so we'll clip the read to the portion of the
+	 * buffer within bsize and zero out the remainder.
+	 */
+	if (dnode->dn_maxblkid == 0) {
+		size_t newbuflen;
+
+		newbuflen = offset > bsize ? 0 : MIN(buflen, bsize - offset);
+		bzero((char *)buf + newbuflen, buflen - newbuflen);
+		buflen = newbuflen;
+	}
+
 	/*
 	 * Note: bsize may not be a power of two here so we need to do an
 	 * actual divide rather than a bitshift.