fnctl: f_modown should call write_lock_irqsave/restore

fnctl: f_modown should call write_lock_irqsave/restore

Commit 703625118069f9f8960d356676662d3db5a9d116 exposed that f_modown()
should call write_lock_irqsave instead of just write_lock_irq so that
because a caller could have a spinlock held and it would not be good to
renable interrupts.

Cc: Eric W. Biederman <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Alan Cox <[email protected]>
Cc: Tavis Ormandy <[email protected]>
Cc: stable <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>