Btrfs: do not call file_update_time in aio_write

This starts a transaction and dirties the inode everytime we call it, which
is super expensive if you have a write heavy workload.  We will be updating
the inode when the IO completes and we reserve the space for the inode
update when we reserve space for the write, so there is no chance of loss of
information or enospc issues.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>