LU-2782 mdt: add macros for fid string len

LU-2782 mdt: add macros for fid string len

add 2 macros for the length of a fid string 0xSEQ:0xOID:0xVER
and it's brace version (FID_NOBRACE_LEN, and FID_LEN)

Signed-off-by: JC Lafoucriere <jacques-charles.lafoucriere@cea.fr>
Change-Id: If257b7e12f43ea48bbdf6a6788d285d465c6a1bc
Reviewed-on: http://review.whamcloud.com/5299
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>