Suppress cppcheck nullPointer error in zfs_write

Newer versions of cppcheck find the potential NULL pointer
bug in zfs_write(). The function is difficult to refactor without
extensive work, so suppress the potential NULL pointer error
which cannot occur for now.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Closes #5882