sys/linux: add descriptions for BSG devices

BSG is a block layer version of SG driver with its own devices,
which can be found in /dev/bsg/*. Currently, syzkaller barely touches
related code in block/ and drivers/scsi/ source directories,
so update the descriptions to nudge the fuzzer in the right direction.

Specifically,
- create a separate description file dev_bsg.txt;
- move openat$bsg from sys.txt and fix the way devices
  in question are accessed;
- describe necessary syscalls and structs, most importantly, sg_io_v4.
- add a few TODOs to address later.

A few words about flaws in sq_io_v4 description:
Some fields were left more ambigious than desired. Once more research
into the way bsg operates is done, as well as related coverage is
gathered, those flaws will be corrected.

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