zfs_alloc/zfs_free were meant to be for temporary allocations, but they were occasionally used for more long-lasting allocations (e.g. vdev_raidz_map_alloc). This has problems, as one must carefully free() memory in the exact opposite order that it was allocated (size-wise) due to the error checking done by zfs_free (freed_ptr == zfs_temp_ptr - sz, i.e. removing sz bytes from our block of memory better put us back at the address we're freeing).

Rectify this by removing them and just replacing uses with malloc/free. We weren't gaining much from the panic anyways, and it's error-prone.