sysfs: collapse fs/sysfs/bin.c::fill_read() into read()

read() is simple enough and fill_read() being in a separate function
doesn't add anything.  Let's collapse it into read().  This will make
merging bin file handling with regular file.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>