When reading/writing block devices, Cygwin emulates Linux, providing
a byte-exact file position, albeit the underlying device drivers don't.
Unfortunately this only worked correctly for reading. The raw_write
method failed to revalidate the buffer after the read-modify-write
cycle in case len is not a multiple of the sector length. This in
turn resulted in lseek reporting a wrong file pointer.
Also, fix a condition for invalidating the buffer after writing from
a remaining read buffer.
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>
