staging/lustre/ptlrpc: Fix potential NULL pointer dereference

In lov_unpackmd() there's this strange bit of code where we first try
to look inside of lmm striping pattern for it's type, and then
we check if the pattern is NULL which cannot be right.
Move the check under if (lmm) branch so that it's safe.

Found by Coverity version 6.6.1

Signed-off-by: Sebastien Buisson <sebastien.buisson@bull.net>
Reviewed-on: http://review.whamcloud.com/7827
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-4049


Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: jacques-Charles Lafoucriere <jacques-charles.lafoucriere@cea.fr>
Signed-off: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>