dax/bus.c: don't use down_write_killable for non-user processes

dax/bus.c: don't use down_write_killable for non-user processes

Change an instance of down_write_killable() to a simple down_write() where
there is no user process that might want to interrupt the operation.

Link: https://lkml.kernel.org/r/20240430-vv-dax_abi_fixes-v3-3-e3dcd755774c@intel.com
Fixes: c05ae9d85b47 ("dax/bus.c: replace driver-core lock usage by a local rwsem")
Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
Reported-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Cc: Alison Schofield <alison.schofield@intel.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>