common: support black listing fs in _supported_fs()

common: support black listing fs in _supported_fs()

For example:
_supported_fs ^xfs

There is no need to specify "generic" when using a block list
"all other fs are supported" is implied.

Converted some generic tests that open code this condition without
emitting a meaningful reason.

More generic test like generic/186,187 could use a block list, but
were not converted because they emit some meaningful reason:
  _notrun "Can't fragment free space on btrfs."

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>