Fix maybe uninitialized variable warning

Commit 1c2358c12 restructured this code and introduced a warning
about the variable maybe not being initialized.  This cannot happen
with the updated code but we should initialize the variable anyway
to silence the warning.

    zpl_file.c: In function ‘zpl_iter_write’:
    zpl_file.c:324:9: warning: ‘count’ may be used uninitialized
        in this function [-Wmaybe-uninitialized]

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #11373

module/os/linux/zfs/zpl_file.c

@@ -318,7 +318,7 @@ zpl_iter_write(struct kiocb *kiocb, struct iov_iter *from)
 	struct file *filp = kiocb->ki_filp;
 	struct inode *ip = filp->f_mapping->host;
 	uio_t uio;
-	size_t count;
+	size_t count = 0;
 	ssize_t ret;
 
 	ret = zpl_generic_write_checks(kiocb, from, &count);