libfstools: fix multiple volume_identify usages with the same volume

This fixes e.g. factory-flashed startup issue with jffs2 on ubi overlay

Commit ba019965 ("libfstools: accept volume as argument in most calls")
broke startup for factory-flashed jffs2 on ubi systems, causing substantial
slowdown in factory environments.

When starting up with a factory-flashed jffs2 on ubi system, the "rootfs_data"
volume contains a deadcode marker. In the start phase, mount_root then mounts a
tmpfs overlay, and postpones remounting of the jffs2 overlay until the done
phase of the startup.

The refactoring in ba019965 eliminated an unneeded call to volume_find() when
done() called jffs2_switch(). Unfortunately the refactoring did not take into
account that volume_identify() does not function correctly when called twice in
a row on the same struct volume when using an mtd driver.

mtd_volume_identify() uses mtd_volume_load() to open an fd to the mtd device
and reads a potential deadcode marker from the fd. The first time this works,
and FS_DEADCODE is returned.

When volume_identify() is called a second time however, mtd_volume_load()
notices that we already have an open fd, does nothing further and returns 0
without resetting the file offset to 0. mtd_volume_identify() now reads past
the deadcode marker and now returns FS_JFFS2 if the mtd device is a UBIVOLUME.

jffs2_switch() then handles the wrong case, either pulling the root out from
under user-space in Chaos Calmer, or indefinitely sticking to a tmpfs overlay
in later OpenWRT builds.

Signed-off-by: Pieter Smith <pieter.smith@philips.com>
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>