posix-clocks: Check write permissions in posix syscalls

pc_clock_settime() and pc_clock_adjtime() do not check whether the fd
was opened in write mode, so a clock can be set with a read only fd.

[ tglx: We deliberately do not return -EPERM as we want this to be
  	distingushable from the capability based permission check ]

Signed-off-by: Torben Hohn <torbenh@gmx.de>
LKML-Reference: <1299173174-348-4-git-send-email-torbenh@gmx.de>
Cc: Richard Cochran <richard.cochran@omicron.at>
Cc: John Stultz <johnstul@us.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>