fs/coredump: use kmap_local_page()

In dump_user_range() there is no reason for the mapping to be global.  Use
kmap_local_page() rather than kmap.

Link: https://lkml.kernel.org/r/20210203223328.558945-1-ira.weiny@intel.com


Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>