ext4: fix out-of-date comments in extents.c

In this patch, ext4_ext_try_to_merge has been change to merge 
an extent both left and right.  So we need to update the comment
in here.

Signed-off-by: HaiboLiu <HaiboLiu6@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>