btrfs: use btrfs_warn() to log message at btrfs_add_extent_mapping()

At btrfs_add_extent_mapping(), if we failed to merge the extent map, which
is unexpected and theoretically should never happen, we use WARN_ONCE() to
log a message which is not great because we don't get information about
which filesystem it relates to in case we have multiple btrfs filesystems
mounted. So change this to use btrfs_warn() and surround the error check
with WARN_ON() so we always get a useful stack trace and the condition is
flagged as "unlikely" since it's not expected to ever happen.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>