Turned out some VMware userspace does pread(2) on /proc/uptime, but
seqfiles currently don't allow pread() resulting in -ESPIPE.

Seqfiles in theory can do pread(), but this can be a long story,
so revert to ->read_proc until then.

http://bugzilla.kernel.org/show_bug.cgi?id=11856Signed-off-by: NAlexey Dobriyan <adobriyan@gmail.com>

and move it to fs/proc/uptime.c while I'm at it.
Signed-off-by: NAlexey Dobriyan <adobriyan@gmail.com>