powerpc/kexec: fix physical address calculation in clear_utlb_entry()

In relocate_32.S, function clear_utlb_entry() goes into real mode. To
do so, it has to calculate the physical address based on the virtual
address. To get the virtual address it uses 'bl' which is problematic
(see commit c974809a26a1 ("powerpc/vdso: Avoid link stack corruption
in __get_datapage()")). In addition, the calculation is done on a
wrong address because 'bl' loads LR with the address of the following
instruction, not the address of the target. So when the target is not
the instruction following the 'bl' instruction, it may lead to
unexpected behaviour.

Fix it by re-writing the code so that is goes via another path which
is based 'bcl 20,31,.+4' which is the right instruction to use for that.

Fixes: 683430200315 ("powerpc/47x: Kernel support for KEXEC")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/dc4f9616fba9c05c5dbf9b4b5480eb1c362adc17.1741256651.git.christophe.leroy@csgroup.eu

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