_check_generic_filesystem support for fsck parameters

_check_generic_filesystem support for fsck parameters

_check_generic_filesystem now support fsck parameters. Fsck parameters
can be passed by FSCK_OPTIONS environmental variable. Default values
will be used if FSCK_OPTIONS is missing or empty (same mechanism as
MOUNT_OPTIONS and MKFS_OPTIONS).

_check_extN_filesystem function is no longer needed, extN filesystem are
properly handled by default values in _check_generic_filesystem.