LU-3333 ptlrpc: Protect request buffer changing

LU-3333 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

Change-Id: I0fa690aabd8696a9f05b94c66e06e30eefb5c759
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Reviewed-on: http://review.whamcloud.com/10074
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>