IB/ucm: fix UCM link error

Building UCM with CONFIG_INFINIBAND_USER_ACCESS=m results in a
set of link errors including:

drivers/infiniband/core/ucm.o: In function `ib_ucm_event_handler':
ucm.c:(.text+0x6dc): undefined reference to `ib_copy_path_rec_to_user'
drivers/infiniband/core/ucma.o: In function `ucma_event_handler':
ucma.c:(.text+0xdc0): undefined reference to `ib_copy_ah_attr_to_user'

To get it to build-test again, this makes the option itself a
tristate, which lets Kconfig figure out the dependency correctly.

Fixes: 486edfb1

 ("IB/ucm: Fix compiling ucm.c")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>