btrfs-progs: get sparse checking working

btrfs-progs: get sparse checking working

When I run sparse checking it gives the following error:

 $ make C=1 V=1
 gcc -MM -MG -MF cmds-fi-usage.o.d -MT cmds-fi-usage.o -MT \
 cmds-fi-usage.static.o -MT cmds-fi-usage.o.d -g -O1 -Wall \
 -D_FORTIFY_SOURCE=2 -include config.h -DBTRFS_FLAT_INCLUDES \
 -D_XOPEN_SOURCE=700 -fno-strict-aliasing -fPIC   cmds-fi-usage.c
    [SP]     ctree.c
 sparse -g -O1 -Wall -D_FORTIFY_SOURCE=2 -include config.h \
 -DBTRFS_FLAT_INCLUDES -D_XOPEN_SOURCE=700 -fno-strict-aliasing -fPIC \
 -include  -D__CHECKER__ -D__CHECK_ENDIAN__ -Wbitwise -Wuninitialized \
 -Wshadow -Wundef -U_FORTIFY_SOURCE ctree.c
 builtin:1:15: error: unable to open '-D__CHECKER__'
 Makefile:177: recipe for target 'ctree.o' failed
 make: *** [ctree.o] Error 1

It means '$(check_defs)' is passed to sparse as NULL and looks
'$(check_defs)'should be assigned before assigning '$(CHECKER_FLAGS)'.
BTW, I'm not familiar with make tool so there could be some my
misunderstanding and better solution.

Signed-off-by: Byongho Lee <bhlee.kernel@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.com>