ext4: add corruption check in ext4_xattr_set_entry()

ext4: add corruption check in ext4_xattr_set_entry()

commit 5369a762c882c0b6e9599e4ebbb3a9ba9eee7e2d upstream.

In theory this should have been caught earlier when the xattr list was
verified, but in case it got missed, it's simple enough to add check
to make sure we don't overrun the xattr buffer.

This addresses CVE-2018-10879.

https://bugzilla.kernel.org/show_bug.cgi?id=200001

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Cc: stable@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 0dc148230f3827f49b1e2a72d14cbad0c5c63e86)

Orabug: 29437127
CVE: CVE-2018-10879

Signed-off-by: John Donnelly <John.P.Donnelly@oracle.com>
Reviewed-by: Jack Vogel <jack.vogel@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>
Conflicts:
	fs/ext4/xattr.c

Signed-off-by: Brian Maly <brian.maly@oracle.com>