RDMA: Fix building with CONFIG_MMU=n

The zap_vma_ptes() is declared but not defined on NOMMU kernels, causing a
link error for the newly added uverbs code:

drivers/infiniband/core/uverbs_main.o: In function `uverbs_user_mmap_disassociate':
uverbs_main.c:(.text+0x114c): undefined reference to `zap_vma_ptes'
drivers/infiniband/core/uverbs_main.o: In function `rdma_umap_open':
uverbs_main.c:(.text+0x53c): undefined reference to `zap_vma_ptes'

Since all user access for all of our drivers depend on remapping pages to
user space disable USER_ACCESS when there is no mmu.

Fixes: 5f9794dc ("RDMA/ucontext: Add a core API for mmaping driver IO memory")
Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>