fs: update fs_dev_part in fs_set_blk_dev_with_part()

As in the case of fs_set_blk_dev(), fs_set_blk_dev_with_part() should
maintain and update fs_dev_part whenever called.

Without this patch, a problem will come up when an efi binary associated
with efi's BOOTxxxx variable is invoked via "bootefi bootmgr".

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Signed-off-by: Alexander Graf <agraf@suse.de>