proc: revert /proc/uptime to ->read_proc hook

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=11856

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>