powerpc: Add __must_check to set_memory_...()

After the following powerpc commits, all calls to set_memory_...()
functions check returned value.
- Commit 8f17bd2f4196 ("powerpc: Handle error in mark_rodata_ro() and
mark_initmem_nx()")
- Commit f7f18e30 ("powerpc/kprobes: Handle error returned by
set_memory_rox()")
- Commit 009cf11d4aab ("powerpc: Don't ignore errors from
set_memory_{n}p() in __kernel_map_pages()")
- Commit 9cbacb83 ("powerpc: Don't ignore errors from
set_memory_{n}p() in __kernel_map_pages()")
- Commit 78cb0945 ("powerpc: Handle error in mark_rodata_ro() and
mark_initmem_nx()")

All calls in core parts of the kernel also always check returned value,
can be looked at with following query:

  $ git grep -w -e set_memory_ro -e set_memory_rw -e set_memory_x -e set_memory_nx -e set_memory_rox `find . -maxdepth 1 -type d | grep -v arch | grep /`

It is now possible to flag those functions with __must_check to make
sure no new unchecked call it added.

Link: https://github.com/KSPP/linux/issues/7


Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://patch.msgid.link/775dae48064a661554802ed24ed5bdffe1784724.1725723351.git.christophe.leroy@csgroup.eu