LU-2951 mdt: Increase bc_req_max_size for MDS_REQUEST_PORTAL

Large EA tests triggered error messages like this on MDSs:

  10:38:21:LNetError: 3022:0:(lib-ptl.c:190:lnet_try_match_md())
  Matching packet from 12345-10.10.17.9@tcp, match 1429230968490588
  length 65928 too big: 117674 left, 49386 allowed

These were the REINT_SETXATTR requests carrying large EA values.  They
were dropped because the MDSs did not expect request buffers larger
than 49386 bytes.  This patch increases bc_req_max_size (and
bc_rep_max_size) for MDS_REQUEST_PORTAL (and MDS_REPLY_PORTAL) to
accommodate REINT_SETXATTR requests (and MDS_GETXATTR replies) with
255-byte names and 65536-byte values (or 65536-byte lists of EA
names).

Change-Id: Ifdcda6d3e91aa6115e5cafd5abd6c89b15485020
Signed-off-by: Li Wei <wei.g.li@intel.com>
Reviewed-on: http://review.whamcloud.com/5703
Tested-by: Hudson
Reviewed-by: Liang Zhen <liang.zhen@intel.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>