Btrfs: fix off-by-one error of the reserved size of btrfs_allocate()

alloc_end is not the real end of the current extent, it is the start of the
next adjoining extent. So we needn't +1 when calculating the size the space
that is about to be reserved.

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Reviewed-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>