ETXTBSY doesn't have a direct anaolog in NFS, so just map it to nfserr_io.

As this is the default error code this does not change the operation of nfsd. 
It only reduces logging.
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>