uptime: fix a problem with subsequent reads.

uptime: fix a problem with subsequent reads.

When doing subsequent reads of uptime on an open file handle
in the form:

read
lseek 0L, SEEK_SET
read

the second (and later) reads cause that the error
"failed to write to cache" was printed. This
happens for example with "top". top would print the error:

bad data in /proc/uptime

To fix this problem use the whole size of the buffer instead of the d->size
because this is set on the first read.

This behavior was introduced with commit 0ecddf023a4caf8e8d2fe7e9125d777a06c5ec12.

Signed-off-by: Bernhard Miklautz <bernhard.miklautz@shacknet.at>