In the v5.4 merge window, a cleanup patch from Al Viro conflicted
with my rework of the compat handling for sg.c read(). Linus Torvalds
did a correct merge but pointed out that the resulting code is still
unsatisfactory.
I later noticed that the sg_new_read() function still gets the compat
mode wrong, when the 'count' argument is large enough to pass a
compat_sg_io_hdr object, but not a nativ sg_io_hdr.
To address both of these, move the definition of compat_sg_io_hdr
into a scsi/sg.h to make it visible to sg.c and rewrite the logic
for reading req_pack_id as well as the size check to a simpler
version that gets the expected results.