introduce write_trylock_irqsave()

Introduce a write_trylock_irqsave() implementation.  Similar in style to
the implementation of spin_trylock_irqsave() in mainline.

Signed-off-by: Satyam Sharma <ssatyam@cse.iitk.ac.in>
Cc: Sripathi Kodi <sripathik@in.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>