fs: ext4: Unmount FS in do_fs_type()

Unlike other generic FS accessors, fs_get_info() does not call fs_close()
at the end of it's operation. Thus, using fs_get_info() in do_fs_type()
without calling fs_close() causes potential memory leak by creating new
filesystem structures on each call of do_fs_type().

The test case to trigger this problem is as follows. It is required to
have ext4 filesystem on the first partition of the SDMMC device, since
ext4 requires stateful mount and causes memory allocation.
=> while true ; do mmc rescan ; fstype mmc 1 ; done
Eventually, the mounting of ext4 will fail due to malloc failures
and the filesystem will not be correctly detected.

This patch fixes the problem by adding the missing fs_close().

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>