Btrfs: fix wrong i_size when truncating a file to a larger size

Btrfsck report error 100 after the 83th case of xfstests was run, it means
the i_size of the file is wrong.

The reason of this bug is that:
Btrfs increased i_size of the file at the beginning, but it failed to expand
the file, and failed to update the i_size to the old size because there is no
enough space in the file system, so we found a wrong i_size.

This patch fixes this bug by updating the i_size just when we pass the file
expanding and get enough space to update i-node.

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>