extent scan: log the bfr when removing a prealloc extent

extent scan: log the bfr when removing a prealloc extent

With subvol scan, the crawl task name is the subvol/inode pair
corresponding to the file offset in the log message.  The identity of
the file can be determined by looking up the subvol/inode pair in the
log message.

With extent scan, the crawl task name is the extent bytenr corresponding
to the file offset in the log message.  This extent is deleted when the
log message is emitted, so a later lookup on the extent bytenr will not
find any references to the extent, and the identity of the file cannot
be determined.

Log the bfr, which does a /proc lookup on the name of the fd, so the
filename is logged.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>