coredump: add DAX filtering for ELF coredumps

Add two new flags to the existing coredump mechanism for ELF files to
allow us to explicitly filter DAX mappings.  This is desirable because
DAX mappings, like hugetlb mappings, have the potential to be very
large.

Update the coredump_filter documentation in
Documentation/filesystems/proc.txt so that it addresses the new DAX
coredump flags.  Also update the documented default value of
coredump_filter to be consistent with the core(5) man page.  The
documentation being updated talks about bit 4, Dump ELF headers, which
is enabled if CONFIG_CORE_DUMP_DEFAULT_ELF_HEADERS is turned on in the
kernel config.  This kernel config option defaults to "y" if both ELF
binaries and coredump are enabled.

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Acked-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
diff --git a/include/linux/sched.h b/include/linux/sched.h
index b7b9501..3c02d92 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -483,9 +483,11 @@
 #define MMF_DUMP_ELF_HEADERS	6
 #define MMF_DUMP_HUGETLB_PRIVATE 7
 #define MMF_DUMP_HUGETLB_SHARED  8
+#define MMF_DUMP_DAX_PRIVATE	9
+#define MMF_DUMP_DAX_SHARED	10
 
 #define MMF_DUMP_FILTER_SHIFT	MMF_DUMPABLE_BITS
-#define MMF_DUMP_FILTER_BITS	7
+#define MMF_DUMP_FILTER_BITS	9
 #define MMF_DUMP_FILTER_MASK \
 	(((1 << MMF_DUMP_FILTER_BITS) - 1) << MMF_DUMP_FILTER_SHIFT)
 #define MMF_DUMP_FILTER_DEFAULT \