ext4: don't use EXT4_FREE_BLOCKS_FORGET unnecessarily

Commit 18888cf0: "ext4: speed up truncate/unlink by not using
bforget() unless needed" removed the use of EXT4_FREE_BLOCKS_FORGET in
the most important codepath for file systems using extents, but a
similar optimization also can be done for file systems using indirect
blocks, and for the two special cases in the ext4 extents code.

Cc: Andrey Sidorov <qrxd43@motorola.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>