LU-1821 build: fix build error about VLA at file scope

LU-1821 build: fix build error about VLA at file scope

Fix build error:
drivers/incore/fs_incore.c:340:54: error: variably modified
‘incore_dir_template’ at file scope [-Werror]

C99 does not allow VLA at file scope. Allocate incore_dir_template
dynamically to avoid build error with newer gcc(>4.6.3) that checks
this.

Signed-off-by: Peng Tao <tao.peng@emc.com>
Change-Id: Icc81c649f58772e76e1cd3dcd42348a53a9a942b
Reviewed-on: http://review.whamcloud.com/3856
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>