bpf: Remove custom build rule

According to the documentation, when building a kernel with the C=2
parameter, all source files should be checked. But this does not happen
for the kernel/bpf/ director

bpf: Remove custom build rule

According to the documentation, when building a kernel with the C=2
parameter, all source files should be checked. But this does not happen
for the kernel/bpf/ directory.

$ touch kernel/bpf/core.o
$ make C=2 CHECK=true kernel/bpf/core.o

Outputs:

CHECK scripts/mod/empty.c
CALL scripts/checksyscalls.sh
DESCEND objtool
INSTALL libsubcmd_headers
CC kernel/bpf/core.o

As can be seen the compilation is done, but CHECK is not executed. This
happens because kernel/bpf/Makefile has defined its own rule for
compilation and forgotten the macro that does the check.

There is no need to duplicate the build code, and this rule can be
removed to use generic rules.

Acked-by: Masahiro Yamada <[email protected]>
Tested-by: Oleg Nesterov <[email protected]>
Tested-by: Alan Maguire <[email protected]>
Signed-off-by: Alexey Gladkov <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

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