configure: don't check for fls

fls should never be provided by system headers.  It seems like on MacOS
it did, but as we're not supporting MacOS anymore there is no need to
check for it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>