Btrfs: be more polite in the async caching threads

The semaphore used by the async caching threads can prevent a
transaction commit, which can make the FS appear to stall.  This
releases the semaphore more often when a transaction commit is
in progress.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

diff --cc fs/btrfs/extent-tree.c
Simple merge

diff --cc fs/btrfs/transaction.c
Simple merge

diff --cc fs/btrfs/transaction.h
Simple merge