Set file mode during zfs_write

Apply https://github.com/openzfs/zfs/pull/11576

Direct commit from upstream openzfs. Full commit message below:

Set file mode during zfs_write

3d40b65 refactored zfs_vnops.c, which shared much code verbatim between
Linux and BSD. After a successful write, the suid/sgid bits are reset,
and the mode to be written is stored in newmode. On Linux, this was
propagated to both the in-memory inode and znode, which is then updated
with sa_update.

3d40b65 accidentally removed the initialization of newmode, which
happened to occur on the same line as the inode update (which has been
moved out of the function).

The uninitialized newmode can be saved to disk, leading to a crash on
stat() of that file, in addition to a merely incorrect file mode.

Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Antonio Russo <aerusso@aerusso.net>
Closes #11474
Closes #11576

Obtained from: openzfs/zfs@f8ce8aed0
MFC after: 0 days
Sponsored by: iXsystems, Inc.

(cherry picked from commit e9d419a05357036ea2fd37218d853d2c713d55cc)