RomFS should advance the destination buffer pointer when reading data from a
blockdev source (the data may be split over multiple blocks, each requiring its
own sb_read() call).  Without this, all the data is copied to the beginning of
the output buffer.
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Tested-by: NMichal Simek <monstr@monstr.eu>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>