MFC r328342:

Add SPDX identifiers to linux_ptrace.c and cfumass.c.

MFC r314290: Implement use of multiple transfers per I/O.

This change removes limitation of single S/G list entry and limitation on
maximal I/O size, using multiple data transfers per I/O if needed. Also
it removes code duplication between send and receive paths, which are now
completely equal.

MFC r314256: Use ctl_queue_sense() to implement sense data reporting.

USB MS BBB transport does not support autosense, so we have to queue any
sense data back to CTL for later fetching via REQUEST SENSE.

MFC r314254: Update kern_data_resid according to r312291.

This now mandatory for correct operation.

MFC r313959:

Add USB Mass Storage CTL frontend. This makes it possible
for USB OTG-capable hardware to implement device side of USB
Mass Storage, ie pretend it's a flash drive. It's configured
in the same way as other CTL frontends, using ctladm(8)
or ctld(8). Differently from usfs(4), all the configuration
can be done without rebuilding the kernel.

Testing and review is welcome. Right now I'm still moving,
and I don't have access to my test environment, so I'm somewhat
reluctant to making larger changes to this code; on the other
hand I don't want to let it sit on Phab until my testing setup
is back, because I want to get it into 11.1-RELEASE.

Relnotes: yes
Sponsored by: The FreeBSD Foundation