ext4: speed up truncate/unlink by not using bforget() unless needed

Do not iterate over data blocks scanning for bh's to forget as they're
never exist. This improves time taken by unlink / truncate syscall.
Tested by continuously truncating file that is being written by dd.
Another test is rm -rf of linux tree while tar unpacks it. With
ordered data mode condition unlikely(!tbh) was always met in
ext4_free_blocks. With journal data mode tbh was found only few times,
so optimisation is also possible.

Unlinking fallocated 60G file after doing sync && echo 3 >
/proc/sys/vm/drop_caches && time rm --help

X86 before (linux 3.6-rc4):
# time rm -f test1
real    0m2.710s
user    0m0.000s
sys     0m1.530s

X86 after:
# time rm -f test1
real    0m0.644s
user    0m0.003s
sys     0m0.060s

MIPS before (linux 2.6.37):
# time rm -f test1
real    0m 4.93s
user    0m 0.00s
sys     0m 4.61s

MIPS after:
# time rm -f test1
real    0m 0.16s
user    0m 0.00s
sys     0m 0.06s
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Andrey Sidorov <qrxd43@motorola.com>

fs/ext4/extents.c

@@ -2318,10 +2318,13 @@ static int ext4_remove_blocks(handle_t *handle, struct inode *inode,
 	struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
 	unsigned short ee_len =  ext4_ext_get_actual_len(ex);
 	ext4_fsblk_t pblk;
-	int flags = EXT4_FREE_BLOCKS_FORGET;
+	int flags = 0;
 
 	if (S_ISDIR(inode->i_mode) || S_ISLNK(inode->i_mode))
-		flags |= EXT4_FREE_BLOCKS_METADATA;
+		flags |= EXT4_FREE_BLOCKS_METADATA | EXT4_FREE_BLOCKS_FORGET;
+	else if (ext4_should_journal_data(inode))
+		flags |= EXT4_FREE_BLOCKS_FORGET;
+
 	/*
 	 * For bigalloc file systems, we never free a partial cluster
 	 * at the beginning of the extent.  Instead, we make a note