Off by one fix for btrfs_drop_extents

One of my old patches introduces a new bug to
btrfs_drop_extents(changeset 275). Inline extents are not truncated
properly when "extent_end == end", it can trigger the BUG_ON at
file.c:600.  I hope I don't introduce new bug this time.

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Signed-off-by: Chris Mason <chris.mason@oracle.com>