Btrfs: make find_first_extent_bit be able to cache any state

Right now the only caller of find_first_extent_bit() that is interested
in caching extent states (transaction or log commit), never gets an extent
state cached. This is because find_first_extent_bit() only caches states
that have at least one of the flags EXTENT_IOBITS or EXTENT_BOUNDARY, and
the transaction/log commit caller always passes a tree that doesn't have
ever extent states with any of those flags (they can only have one of the
following flags: EXTENT_DIRTY, EXTENT_NEW or EXTENT_NEED_WAIT).

This change together with the following one in the patch series (titled
"Btrfs: avoid returning -ENOMEM in convert_extent_bit() too early") will
help reduce significantly the chances of calls to convert_extent_bit()
fail with -ENOMEM when called from the transaction/log commit code.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>
diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index 04dbc80..16c704b 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -883,6 +883,7 @@
 			werr = err;
 		else if (wait_writeback)
 			werr = filemap_fdatawait_range(mapping, start, end);
+		free_extent_state(cached_state);
 		cached_state = NULL;
 		cond_resched();
 		start = end + 1;
@@ -927,6 +928,8 @@
 			err = filemap_fdatawait_range(mapping, start, end);
 		if (err)
 			werr = err;
+		free_extent_state(cached_state);
+		cached_state = NULL;
 		cond_resched();
 		start = end + 1;
 	}