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/[email protected]
Fixes: 2251334 ("rdma/siw: application buffer management")
Signed-off-by: Arnd Bergmann <[email protected]>
Suggested-by: Jason Gunthorpe <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Bernard Metzler <[email protected]>
Cc: "Matthew Wilcox (Oracle)" <[email protected]>
Cc: "Kirill A. Shutemov" <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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 *);