bpf: Use ERR_CAST instead of ERR_PTR(PTR_ERR(...))

bpf: Use ERR_CAST instead of ERR_PTR(PTR_ERR(...))

Intel linux test robot reported a warning that ERR_CAST can be used
for error pointer casting instead of more-complicated/rarely-used
ERR_PTR(PTR_ERR(...)) style.

There is no functionality change, but still let us replace two such
instances as it improves consistency and readability.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202507201048.bceHy8zX-lkp@intel.com/
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://patch.msgid.link/20250720164754.3999140-1-yonghong.song@linux.dev