KVM: MMU: Rename the walk label in walk_addr_generic()

The current name does not explain the meaning well.  So give it a better
name "retry_walk" to show that we are trying the walk again.

This was suggested by Ingo Molnar.

Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>