samples/bpf: Define MAX_ENTRIES instead of a magic number in offwaketime

Define MAX_ENTRIES instead of using 10000 as a magic number in various
places.

Signed-off-by: Muhammad Falak R Wani <falakreyaz@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210815065013.15411-1-falakreyaz@gmail.com