powerpc/bpf: Use bctrl for making function calls

blrl corrupts the link stack. Instead use bctrl when making function
calls from BPF programs.

Reported-by: Anton Blanchard <anton@ozlabs.org>
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/20210609090024.1446800-1-naveen.n.rao@linux.vnet.ibm.com