integck.c: rework file_check_data to immediately dump the buffer containing the errors

integck.c: rework file_check_data to immediately dump the buffer containing the errors

See my problem description int the previous commit, the point is that integck
in file_check_data reads a buffer, and then checks if the data is correct,  it
will do a seek(0), and reread from the same fd. The point is that in the
scenario I observed integck failed (due to a buffer mismatch) but the it saved
(and what was in flash) was actually correct. So I modified this function to
dump the buffers to stderr at the moment an error is found.

Signed-off-by: Elie De Brauwer <eliedebrauwer@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>