os/bluestore: avoid duplicated searches during allocating free extents

os/bluestore: avoid duplicated searches during allocating free extents

When StupidAllocator searches for free extents, if hint is not 0(most times),
it will search extents starting from hint and up in each applicable bin,
if not found, then search all extents in those bins again.

If the extent to find(>= needs) exists below hint and is not in the choose_bin(),
let's say it's in the bin_X; then during this twice searches, all extents from
hint and up in bins before bin_X will be searched twice. This patch will eliminate
those unnecessary searches.

Signed-off-by: Jianjian Huo <samuel.huo@gmail.com>