[PATCH] knfsd: nfs4: hold filp while reading or writing

We're trying to read and write from a struct file that we may not hold a
reference to any more (since a close could be processed as soon as we drop the
state lock).

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>