erofs: fix metabuf leak in inode xattr initialization

In the Linux kernel, the following vulnerability has been resolved: erofs: fix metabuf leak in inode xattr initialization commit bb88e8da0025 ("erofs: use meta buffers for xattr operations") converted xattr operations to use on-stack erofs_buf instances. erofs_init_inode_xattrs() uses such a metabuf while reading the inline xattr header and shared xattr id array. Some error paths after erofs_read_metabuf() leave through out_unlock without dropping the metabuf, so the folio reference can leak. Consolidate the cleanup at out_unlock. erofs_put_metabuf() is a no-op if no folio has been acquired, and this keeps all paths after taking EROFS_I_BL_XATTR_BIT covered by a single cleanup site.