Relax special_small_blocks restrictions

special_small_blocks is applied to blocks after compression, so it
makes no sense to demand its values to be power of 2.  At most
they could be multiple of 512, but that would still buy us nothing,
so lets allow them be any within SPA_MAXBLOCKSIZE.

Also special_small_blocks does not really need to depend on the
set recordsize, enabled pool features or presence of special vdev.
At worst in any of those cases it will just do nothing, so we
should not complicate users lives by artificial limitations.

While there, polish comments for recordsize and volblocksize.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Closes #17497

include/sys/dmu_objset.h

@@ -152,7 +152,7 @@ struct objset {
 	 * The largest zpl file block allowed in special class.
 	 * cached here instead of zfsvfs for easier access.
 	 */
-	int os_zpl_special_smallblock;
+	uint64_t os_zpl_special_smallblock;
 
 	/*
 	 * Pointer is constant; the blkptr it points to is protected by