btrfs: remove duplicate calculation of eb offset in btrfs_bin_search()

btrfs: remove duplicate calculation of eb offset in btrfs_bin_search()

In the main search loop the variable 'oil' (offset in folio) is set
twice, one duplicated when the key fits completely to the contiguous
range. We can remove it and while it's just a simple calculation, the
binary search loop is executed many times so micro optimizations add up.

The code size is reduced by 64 bytes on release config, the loop is
reorganized a bit and a few instructions shorter.

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: David Sterba <dsterba@suse.com>