bios: Don't keep sending BIO_FLUSH after first ENOTSUPP.

When a storage device reports that it does not support cache flush, the
GEOM disk layer by default returns ENOTSUPP in response to a BIO_FLUSH
command.

On AWS, local volumes do not advertise themselves as having write-cache
enabled. When they are selected for L3 on all HDD nodes, the L3
subsystem may inadvertently kick these L3 devices if a BIO_FLUSH command
fails with an ENOTSUPP return code. The fix is to make GEOM disk return
success (0) when this condition occurs and add a sysctl to make this
error handling config-driven

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/710