staging/lustre/ptlrpc: Protect request buffer changing

*_enlarge_reqbuf class of functions can change request body location
for a request that's already in replay list, as such a parallel
traverser of the list (after_reply -> ptlrpc_free_committed) might
access freed and scrambled memory causing assertion.

Since all such users only can get to this request under imp_lock, take
imp_lock to protect against them in *_enlarge_reqbuf

Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Reviewed-on: http://review.whamcloud.com/10074
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3333


Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>