/dev/mem: allow rewinding

commit dcefafb6 ("/dev/mem: dont allow seek to last page") inadvertently
disabled rewinding on /dev/mem.

This broke x86info for example.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Acked-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>