Btrfs: fix double increment of path->slots[0] in btrfs_next_leaf

if 1 is returned by btrfs_search_slot, the path already points to the
first item with 'key > searching key'. So increasing path->slots[0] by
one is superfluous in that case.

Signed-off-by: Yan Zheng <zheng.yan@oracle.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>