nfs: add missing "posix" local_lock constant table definition

"mount -o local_lock=posix..." was broken by the mount API conversion
due to the missing constant.

Fixes: e38bb238

 ("NFS: Convert mount option parsing to use functionality from fs_parser.h")
Signed-off-by: Scott Mayhew <smayhew@redhat.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>