fiemap: Add new extent flag FIEMAP_EXTENT_SHARED

Some filesystems may allow multiple files to point to a particular
extent.  This patch adds flag FIEMAP_EXTENT_SHARED to denote extents
that are shared with other inodes.

Signed-off-by: Sunil Mushran <sunil.mushran@oracle.com>
Acked-by: Mark Fasheh <mfasheh@suse.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>