ocfs2: improve ocfs2 Makefile

Included file path was hard-wired in the ocfs2 makefile, which might
causes some confusion when compiling ocfs2 as an external module.

Say if we compile ocfs2 module as following.
cp -r /kernel/tree/fs/ocfs2 /other/dir/ocfs2
cd /other/dir/ocfs2
make -C /path/to/kernel_source M=`pwd` modules

Acutally, the compiler wil try to find included file in
/kernel/tree/fs/ocfs2, rather than the directory /other/dir/ocfs2.

To fix this little bug, we introduce the var $(src) provided by kbuild.
$(src) means the absolute path of the running kbuild file.

Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Larry Chen <[e-mail]>
Reviewed-by: Andrew Morton <[e-mail]>
Cc: Mark Fasheh <[e-mail]>
Cc: Joel Becker <[e-mail]>
Cc: Junxiao Bi <[e-mail]>
Cc: Joseph Qi <[e-mail]>
Cc: Changwei Ge <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/ocfs2/Makefile

···
 # SPDX-License-Identifier: GPL-2.0
-ccflags-y := -Ifs/ocfs2
+ccflags-y := -I$(src)
 
 obj-$(CONFIG_OCFS2_FS) += 	\
 	ocfs2.o			\

fs/ocfs2/dlm/Makefile

···
-ccflags-y := -Ifs/ocfs2
+ccflags-y := -I$(src)/..
 
 obj-$(CONFIG_OCFS2_FS_O2CB) += ocfs2_dlm.o
 

fs/ocfs2/dlmfs/Makefile

···
-ccflags-y := -Ifs/ocfs2
+ccflags-y := -I$(src)/..
 
 obj-$(CONFIG_OCFS2_FS) += ocfs2_dlmfs.o