f2fs: fix to document inline_xattr_size option

We missed to add document for inline_xattr_size mount option in f2fs.txt,
add it.
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

Documentation/filesystems/f2fs.txt

@@ -126,6 +126,8 @@ disable_ext_identify   Disable the extension list configured by mkfs, so f2fs
                        does not aware of cold files such as media files.
 inline_xattr           Enable the inline xattrs feature.
 noinline_xattr         Disable the inline xattrs feature.
+inline_xattr_size=%u   Support configuring inline xattr size, it depends on
+		       flexible inline xattr feature.
 inline_data            Enable the inline data feature: New created small(<~3.4k)
                        files can be written into inode block.
 inline_dentry          Enable the inline dir feature: data in new created