Fix 2 potential problems:
1. when largest extent needs to be invalidated, it will be reset in
   __drop_largest_extent, which makes __is_extent_same after always
   return false, and largest extent unchanged. Now we update it properly.

2. when extent is split and the latter part remains in tree, next_en
   should be the latter part instead of next extent of original extent.
   It will cause merge failure if there is in-place update, although
   there is not, I think this fix will still makes codes less ambiguous.

This patch also simplifies codes of invalidating extents, and optimizes the
procedues that split extent into two.
There are a few modifications after last patch:
1. prev_en now is updated properly.
2. more codes and branches are simplified.
Signed-off-by: NFan li <fanofcode.li@samsung.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>