Fix handling of errors from dmu_write_uio_dbuf() on FreeBSD

FreeBSD's implementation of zfs_uio_fault_move() returns EFAULT when a
page fault occurs while copying data in or out of user buffers. The VFS
treats such errors specially and will retry the I/O operation (which may
have made some partial progress).

When the FreeBSD and Linux implementations of zfs_write() were merged,
the handling of errors from dmu_write_uio_dbuf() changed such that
EFAULT is not handled as a partial write. For example, when appending
to a file, the z_size field of the znode is not updated after a partial
write resulting in EFAULT.

Restore the old handling of errors from dmu_write_uio_dbuf() to fix
this. This should have no impact on Linux, which has special handling
for EFAULT already.

Reviewed-by: Andriy Gapon <avg@FreeBSD.org>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Mark Johnston <markj@FreeBSD.org>
Closes #12964