xfs: initialize default acls for ->tmpfile()

The current tmpfile handler does not initialize default ACLs. Doing so
within xfs_vn_tmpfile() makes it roughly equivalent to xfs_vn_mknod(),
which is already used as a common create handler.

xfs_vn_mknod() does not currently have a mechanism to determine whether
to link the file into the namespace. Therefore, further abstract
xfs_vn_mknod() into a new xfs_generic_create() handler with a tmpfile
parameter. This new handler calls xfs_create_tmpfile() and d_tmpfile()
on the dentry when called via ->tmpfile().

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>