tty: Fix double grabbing of a spinlock

The HSO changes for kref introduced a recursive spinlock take. All
functions which call put_rxbuf_data already have serial->serial_lock
grabbed.

[Comment to code added-AC]

Signed-off-by: Denis Joseph Barrow <D.Barrow@option.com>
Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>