module: fix MIPS module_layout -> module_memory

Correct the struct's field/member name from mod_mem to mem.

Fixes this build error:
../arch/mips/kernel/vpe.c: In function 'vpe_elfload':
../arch/mips/kernel/vpe.c:643:41: error: 'struct module' has no member named 'mod_mem'
  643 |         v->load_addr = alloc_progmem(mod.mod_mem[MOD_TEXT].size);

Fixes: 2ece476a2346 ("module: replace module_layout with module_memory")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Song Liu <song@kernel.org>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: linux-mips@vger.kernel.org
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>