Btrfs: use mark_inode_dirty when expanding the file

Mark_inode_dirty will call btrfs_dirty_inode which will take care of updating
the inode.  This makes setsize a little cleaner since we don't have to start a
transaction and update the inode in there, we can just call mark_inode_dirty.
Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>