n_tty: Rename tail to old_tail in n_tty_read()

n_tty: Rename tail to old_tail in n_tty_read()

[ Upstream commit 947d66b68f3c4e7cf8f3f3500807b9d2a0de28ce ]

The local tail variable in n_tty_read() is used for one purpose, it
keeps the old tail. Thus, rename it appropriately to improve code
readability.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/22b37499-ff9a-7fc1-f6e0-58411328d122@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Stable-dep-of: 4903fde8047a ("tty: fix hang on tty device with no_room set")
Signed-off-by: Sasha Levin <sashal@kernel.org>