Use the struct file * obtained in nfsd4_open for nfsd_read and nfsd_write when
available.

To do this we add a struct file * argument to nfsd_read and nfsd_write.  If a
struct file is passed in, nfsd_read and nfsd_write will use it, doing just an
access check instead of an open and close.  If the new argument is NULL, they
will fall back on the old behaviour.
Signed-off-by: Andy Adamson <andros@citi.umich.edu>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Neil Brown <neilb@cse.unsw.edu.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>