btrfs: Return EINVAL when length to trim is less than FSB

Currently if len argument in btrfs_ioctl_fitrim() is smaller than
one FSB we will continue and finally return 0 bytes discarded.
However if the length to discard is smaller then file system block
we should really return EINVAL.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>