powerpc64/bpf: Do not save/restore LR on each call to bpf_stf_barrier()

Instead of saving and restoring LR before each invocation to
bpf_stf_barrier(), set SEEN_FUNC flag so that we save/restore LR in
prologue/epilogue.

Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/4446f25478d82a2a4ac9dab2ebdfd88ddf923eb7.1644834730.git.naveen.n.rao@linux.vnet.ibm.com