reiserfs: replace open-coded atomic_dec_and_mutex_lock()

Replace the open-coded logic of atomic_dec_and_mutex_lock() in
reiserfs_file_release().

Link: https://lore.kernel.org/r/20191103094431.GA18576-nikitas.angelinas@gmail.com


Signed-off-by: Nikitas Angelinas <nikitas.angelinas@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>