erofs: unify meta buffers in z_erofs_fill_inode()

There is no need to keep additional local metabufs since we already
have one in `struct erofs_map_blocks`.

This was actually a leftover when applying meta buffers to zmap
operations, see commit 09c54379 ("erofs: use meta buffers for
zmap operations").

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20250716064152.3537457-1-hsiangkao@linux.alibaba.com