dax: Fix incorrect argument passed to xas_set_err()

The argument passed to xas_set_err() to indicate an error should be negative.
Otherwise, xas_error() will return 0, and grab_mapping_entry() will return the
found entry instead of 'SIGBUS' when the entry is not in fact valid.
This would result in problems in subsequent code paths.

Link: https://lore.kernel.org/r/20200729034436.24267-1-lihao2018.fnst@cn.fujitsu.com


Reviewed-by: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Signed-off-by: Hao Li <lihao2018.fnst@cn.fujitsu.com>
Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>