client: use 64-bit value in sync read eof logic

The file size can jump to a value that is very much larger than our current
position (for example, it could be a disk image file that gets a sparse
write at a large offset).  Use a 64-bit value so that 'some' doesn't
overflow.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: John Spray <john.spray@inktank.com>