Merge branch 'bugfix/fatfs_sdmmc_cleanup' into 'master'

fat/sdmmc: unmount FATFS object on error

Failure to call f_mount(NULL,...) makes FATFS attempt to clean up the old
FS object upon next mount. If previous mount operation has failed, some
parts of FS object may not be fully initialized, which will cause errors
(such as attempting to delete a mutex which wasn’t allocated).

Fixes TW11594.

See merge request !679