btrfs: fix file truncation if FALLOC_FL_KEEP_SIZE is specified

Create a small file and fallocate it to a big size with
FALLOC_FL_KEEP_SIZE option, then truncate it back to the
small size again, the disk free space is not changed back
in this case. i.e,

total 4
-rw-r--r-- 1 root root 512 Jun 28 11:35 test

Filesystem      Size  Used Avail Use% Mounted on
....
/dev/sdb1       8.0G   56K  7.2G   1% /mnt

-rw-r--r-- 1 root root 512 Jun 28 11:35 /mnt/test

Filesystem      Size  Used Avail Use% Mounted on
....
/dev/sdb1       8.0G  5.1G  2.2G  70% /mnt

Filesystem      Size  Used Avail Use% Mounted on
....
/dev/sdb1       8.0G  5.1G  2.2G  70% /mnt

With this fix, the truncated up space is back as:
Filesystem      Size  Used Avail Use% Mounted on
....
/dev/sdb1       8.0G   56K  7.2G   1% /mnt

Signed-off-by: Jie Liu <jeff.liu@oracle.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>