remove useless mnt_want_write call in xfs_write

When mnt_want_write was introduced a call to it was added around
xfs_ichgtime, but there is no need for this because a file can't be open
read/write on a r/o mount, and a mount can't degrade r/o while we still
have files open for writing.  As the mnt_want_write changes were never
merged into the CVS tree this patch is for mainline only.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Niv Sardi <xaiki@sgi.com>