drivers/dax/device.c: convert variable to vm_fault_t type

drivers/dax/device.c: convert variable to vm_fault_t type

As part of c2ee30409c37 ("device-dax: Convert to vmf_insert_mixed and
vm_fault_t") in 4.19-rc1, 'rc' was not converted to vm_fault_t.  Now
converted.

Link: http://lkml.kernel.org/r/20180830153813.GA26059@jordon-HP-15-Notebook-PC
Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: Ross Zwisler <zwisler@kernel.org>
Cc: Vishal Verma <vishal.l.verma@intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>