libsa: Fix infinite loop in bzipfs & gzipfs

[dab]

Summary

A bug in the loader's bzipfs & gzipfs filesystems caused compressed
kernel and modules not to work on EFI systems with a veriexec-enabled
loader. Since the size of files in these filesystems are not known
_a priori_ stat would initialize the size to -1 and the loader would
then hang in an infinite loop while trying to seek (read) to the end
of file since the loop termination condition compares the current
offset to that negative target position.

Sponsored by: Dell Technologies | Isilon
MFC to: stable/12, stable/13
MFC after: 1 week

Test Plan

Tested at $WORK. Basically, booting a veriexec-enabled loader with
the current code and compressed kernel or modules would hang. Booting
with compressed kernel or modules and the modified code would not
hang.

[dab]

• Shorten comment

[vangyzen]

stand/libsa/bzipfs.c

344

I believe you mean bzf->bzf_endseen.

[dab]

• Fix typo

[dab]

stand/libsa/bzipfs.c

344

Not sure how that compiled...

[imp]

stand/libsa/bzipfs.c

346

Does this get set with the last readable block of data, or after the readable block of data?

[dab]

stand/libsa/bzipfs.c

346

It is set on the read where the data decompression returns the end-of-stream marker.