Silence 'old_umask' uninit variable warning

Recent changes have caused older versions of gcc to mistakenly
flag 'old_umask' in vn_open() as an unitialized variable. To
silence the warning initialize it.

kernel.c: In function 'vn_open':
kernel.c:525:6: error: 'old_umask' may be used uninitialized
in this function

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>