ANDROID: fix kernelci error in fs/fuse/dir.c

kernelci reported the following error when !CONFIG_FUSE_BPF:

fs/fuse/dir.c:269:40: error: no member named 'backing_inode' in 'struct fuse_inode'

Fixed by wrapping that new code in #ifdef CONFIG_FUSE_BPF.

Fixes: 094905c8 ("ANDROID: fuse-bpf: Always call revalidate for
backing")
Signed-off-by: Todd Kjos <tkjos@google.com>
Change-Id: I08760f340cebf21d3084ca84ac1792bc70f311ff