LU-2489 cfs: Use vfs_write() in cfs_user_write()

LU-2489 cfs: Use vfs_write() in cfs_user_write()

cfs_user_write() calls filp->f_op->write() which does not check the
mode of filp.  Call vfs_write() instead.

Signed-off-by: John L. Hammond <jhammond@tacc.utexas.edu>
Change-Id: I0cbb2e88d813fe53ce9d625c4299fc5277d246f5
Reviewed-on: http://review.whamcloud.com/4824
Reviewed-by: Keith Mannthey <keith.mannthey@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Johann Lombardi <johann.lombardi@intel.com>