WebUI for dataset "Metadata (Special) Small Block Size" is artificially limited to 1 MiB

This issue is the development of the old issues here: and here:

TrueNAS WebUI in the dataset “Edit Options” panel does not allow to set “Metadata (Special) Small Block Size“ even when vfs.zfs.max_recordsize>=special_small_blocks is set. Therefore “Metadata (Special) Small Block Size“ is artificially limited to 1 MiB. As per older cases of artificially limited small block sizes to 128 KiB , then 1 MiB, then in principle - unlimited to 16 MiB (if set in the command line), WebUI limit should be lifted.

Motivation is simple (as was before, described in the above bugs) - it should be allowed, because there are use-cases, where 2-8 MiB “small block sizes” would allow to land whole files on SSD media, instead of HDD. An example would be typical pictures from DSRL cameras, which can be outputed in 2 simultaneous files - small .JPG and big .RAW. JPG often fit as small files to SSD, while RAWs would land on HDD vdevs.

At this moment there is a manual work around, which is inconvenient - if one has to edit dataset options, it must do everything either in the command-line or change small_block_size temporarily to 1 MiB while committing dataset options form, and then set small_block_size in the command line above 1 MiB artifial WebUI limit.