lockd: protect nlm_blocked access in nlmsvc_retry_blocked

In nlmsvc_retry_blocked, the check that the list is non-empty and acquiring
the pointer of the first entry is unprotected by any lock.  This allows a rare
race condition when there is only one entry on the list.  A function such as
nlmsvc_grant_callback() can be called, which will temporarily remove the entry
from the list.  Between the list_empty() and list_entry(),the list may become
empty, causing an invalid pointer to be used as an nlm_block, leading to a
possible crash.

This patch adds the nlm_block_lock around these calls to prevent concurrent
use of the nlm_blocked list.

This was a regression introduced by
f904be9cc77f361d37d71468b13ff3d1a1823dea  "lockd: Mostly remove BKL from
the server".

Cc: Bryan Schumaker <bjschuma@netapp.com>
Cc: stable@vger.kernel.org
Signed-off-by: David Jeffery <djeffery@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
nfsd4: fix minorversion support interface

You can turn on or off support for minorversions using e.g.

	echo "-4.2" >/proc/fs/nfsd/versions

However, the current implementation is a little wonky.  For example, the
above will turn off 4.2 support, but it will also turn *on* 4.1 support.

This didn't matter as long as we only had 2 minorversions, which was
true till very recently.

And do a little cleanup here.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
svcrdma: underflow issue in decode_write_list()

My static checker marks everything from ntohl() as untrusted and it
complains we could have an underflow problem doing:

	return (u32 *)&ary->wc_array[nchunks];

Also on 32 bit systems the upper bound check could overflow.

Cc: stable@vger.kernel.org
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Merge branch 'for-3.11' of git://linux-nfs.org/~bfields/linux

Pull nfsd bugfixes from Bruce Fields:
 "Just three minor bugfixes"

* 'for-3.11' of git://linux-nfs.org/~bfields/linux:
  svcrdma: underflow issue in decode_write_list()
  nfsd4: fix minorversion support interface
  lockd: protect nlm_blocked access in nlmsvc_retry_blocked