LinuxKPI: firmware: add request_partial_firmware_into_buf()

A (so far out-of-tree) driver update needs
request_partial_firmware_into_buf(). Given we load the full .ko file
using firmware(9) just do that and copy the requeste data into the
buffer (rather than poissibly only reading portions of the firmware
file).

(cherry picked from commit fb3c549738bfed0607d95120e5dbf03a62e61b49)