[kernel] apps deadlock on parent bfs image unmount

[diver]

hrev53259 x86_64.

To reproduce:

# create and mount 1G BFS image
dd if=/dev/zero of=1g.bfs bs=1 count=0 seek=1G
mkfs -t bfs -q 1g.bfs 
mkdir /bfs1
mount 1g.bfs
cd /bfs1

# create and mount 100M nested BFS image
dd if=/dev/zero of=100m.bfs bs=1 count=0 seek=100M
mkfs -t bfs -q 100m.bfs 
mkdir /bfs2
mount 100m.bfs

# force unmount parent image
unmount -f /bfs1

would deadlock unmount/Tracker/DriveSetup and result in this syslog output:

KERN: could not read block 2: bytesRead: -1, error: Bad file descriptor
KERN: bfs: Free:484: I/O error
KERN: bfs: Free:1034: I/O error
KERN: could not read block 3: bytesRead: -1, error: Bad file descriptor
KERN: bfs: Free:484: I/O error
KERN: bfs: Free:1034: I/O error
KERN: bfs: could not write log area: Bad file descriptor!
KERN: bfs: writing current log entry failed: I/O error
KERN: could not write back block 16384 (Bad file descriptor)