btrfs: release old extent maps during page release

btrfs: release old extent maps during page release

When removing an extent map at try_release_extent_mapping(), called through
the page release callback (btrfs_releasepage()), we never release an extent
map that is in the list of modified extents. This is to prevent races with
a concurrent fsync using the fast path, which could lead to not logging an
extent created in the current transaction.

However we can safely remove an extent map created in a past transaction
that is still in the list of modified extents (because no one fsynced yet
the inode after that transaction got commited), because such extents are
skipped during an fsync as it is pointless to log them. This change does
that.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>