btrfs: remove noinline attribute from btrfs_cow_block()

It's pointless to have the noiline attribute for btrfs_cow_block(), as the
function is exported and widely used. So remove it.

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