mm: add dummy can_do_mlock() helper

On kernels without CONFIG_MMU, we get a link error for the siw driver:

drivers/infiniband/sw/siw/siw_mem.o: In function `siw_umem_get':
siw_mem.c:(.text+0x4c8): undefined reference to `can_do_mlock'

This is probably not the only driver that needs the function and could
otherwise build correctly without CONFIG_MMU, so add a dummy variant that
always returns false.

Link: http://lkml.kernel.org/r/20190909204201.931830-1-arnd@arndb.de
Fixes: 2251334d ("rdma/siw: application buffer management")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Suggested-by: Jason Gunthorpe <jgg@mellanox.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Bernard Metzler <bmt@zurich.ibm.com>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mm.h

@@ -1405,7 +1405,11 @@ extern void pagefault_out_of_memory(void);
 
 extern void show_free_areas(unsigned int flags, nodemask_t *nodemask);
 
+#ifdef CONFIG_MMU
 extern bool can_do_mlock(void);
+#else
+static inline bool can_do_mlock(void) { return false; }
+#endif
 extern int user_shm_lock(size_t, struct user_struct *);
 extern void user_shm_unlock(size_t, struct user_struct *);