target/user: Fix cast from pointer to phys_addr_t

The uio_mem structure has a member that is a phys_addr_t, but can
be a number of other types too. The target core driver attempts
to assign a pointer from vmalloc() to it, by casting it to
phys_addr_t, but that causes a warning when phys_addr_t is longer
than a pointer:

drivers/target/target_core_user.c: In function 'tcmu_configure_device':
drivers/target/target_core_user.c:906:22: error: cast from pointer to integer of different size [-Werror=pointer-to-int-cast]

This adds another cast to uintptr_t to shut up the warning.
A nicer fix might be to have additional fields in uio_mem
for the different purposes, so we can assign a pointer directly.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>