btrfs: Avoid a rescan for a device which was already not found

btrfs: Avoid a rescan for a device which was already not found

Currently read from missing device triggers rescan. However, it is never
recorded that the device is missing. So, each read of a missing device
triggers rescan again and again. This behavior causes a lot of unneeded
rescans leading to huge slowdowns.

This patch fixes above mentioned issue. Information about missing devices
is stored in the data->devices_attached[] array as NULL value in dev
member. Rescan is triggered only if no information is found for a given
device. This means that only first time read triggers rescan.

The patch drops premature return. This way data->devices_attached[] is
filled even when a given device is missing.

Signed-off-by: Goffredo Baroncelli <kreikack@inwind.it>
Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>