Add a sysctl to get the number of filters available.

sysctl dev.t4nex.<N>.nfilters
sysctl dev.t5nex.<N>.nfilters

MFC after:	3 days
Count the number of hits for a filter by default.

MFC after:	3 days
Remove forced timeout of in-flight commands from mfi_timeout.

While this prevents commands getting stuck forever there is no way to guarantee
that data from the command hasn't been committed to the device.

In addition older mfi firmware has a bug that would cause the controller to
frequently stall IO for over our timeout value, which when combined with
a forced timeout often resulted in panics in UFS; which would otherwise be
avoided when the command eventually completed if left alone.

For reference this timeout issue is resolved in Dell FW package 21.2.1-0000.
Fixed FW package version for none Dell controller will likely vary.

MFC after:	2 days