staging: gs_fpgaboot: add buffer overflow checks

Four fields in struct fpgaimage are char arrays of length MAX_STR (256).
The amount of data read into these buffers is controlled by a length
field in the bitstream file read from userspace. If a corrupt or
malicious firmware file was supplied, kernel data beyond these buffers
can be overwritten arbitrarily.

This patch adds a check of the bitstream's length value to ensure it
fits within the bounds of the allocated buffers. An error condition is
returned from gs_read_bitstream if any of the reads fail.

Signed-off-by: Jacob von Chorus <jacobvonchorus@cwphoto.ca>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>