fs: Fix return in jfs_mkdir and orangefs_mkdir

fs: Fix return in jfs_mkdir and orangefs_mkdir

Return NULL instead of passing to ERR_PTR while err is zero
Fixes these smatch warnings:
  - fs/jfs/namei.c:311 jfs_mkdir() warn: passing zero to 'ERR_PTR'
  - fs/orangefs/namei.c:369 orangefs_mkdir() warn: passing zero
    to 'ERR_PTR'

Fixes: 88d5baf69082 ("Change inode_operations.mkdir to return struct dentry *")
Signed-off-by: Hongling Zeng <zenghongling@kylinos.cn>
Link: https://patch.msgid.link/20260501071058.1243245-1-zenghongling@kylinos.cn
Reviewed-by: Jori Koolstra <jkoolstra@xs4all.nl>
Signed-off-by: Christian Brauner <brauner@kernel.org>