generic: fitblk: close block device if mapping image failed

generic: fitblk: close block device if mapping image failed

In case a broken fit image is present on flash the fitblk driver would
not map any /dev/fit* devices, but also not always close the block device
the image resides on. In case of ubiblock devices this is fatal as one
then cannot remove the ubiblock device (-EBUSY), and hence cannot replace
the broken image.
Always close the block device in case no sub-image was mapped.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
(cherry picked from commit 2b9c81d1cbc65eeb9304b7274058418efa8a9c78)