xfs: fix confusing xfs_extent_item variable names

[ Upstream commit 578c714b ]

Change the name of all pointers to xfs_extent_item structures to "xefi"
to make the name consistent and because the current selections ("new"
and "free") mean other things in C.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Leah Rumancik <leah.rumancik@gmail.com>
Acked-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>