loader: rewrite zfs reader zap code to use malloc

[tsoome]

Summary

First step on removing zfs_alloc.

Test Plan

I have used this implementation since 21 Nov 2019.

[tsoome]

replace if chain by switch. Print out entire zap_block_type.

[delphij]

LGTM. There are a few minor nits but these are optional.

stand/libsa/zfs/zfsimpl.c

2458 ↗

(On Diff #67556)

Nit, optional: 'lp' is passed and from an on-stack variable, we could use it directly, e.g. *lp = malloc(sizeof(**lp)) below.

2609 ↗

(On Diff #67556)

Nit, optional: I think it's better to have something like zap_leaf_free() that free's zl->l_phys and zl.

[tsoome]

Suggestions from delphij, thanks!