NFSv4/pnfs: Allow layoutget to return EAGAIN for softerr mounts

If we're using the 'softerr' mount option, we may want to allow
layoutget to return EAGAIN to allow knfsd server threads to return a
JUKEBOX/DELAY error to the client instead of busy waiting.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>