fs: add fs_get_type() for current filesystem type

This function is a variant of fs_get_type_name() and returns a filesystem
type with which the current device is associated.
We don't want to export fs_type variable directly because we have to take
care of it consistently within fs.c.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>