staging/lustre/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)

Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-2782
Lustre-change: http://review.whamcloud.com/5299


Signed-off-by: JC Lafoucriere <jacques-charles.lafoucriere@cea.fr>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Peng Tao <tao.peng@emc.com>
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>