erofs: clean up header parsing for ztailpacking and fragments

erofs: clean up header parsing for ztailpacking and fragments

[ Upstream commit 540787d38b10dbc16a7d2bc2845752ab1605403a ]

Simplify the logic in z_erofs_fill_inode_lazy() by combining the
handling of ztailpacking and fragments, as they are mutually exclusive.

Note that `h->h_clusterbits >> Z_EROFS_FRAGMENT_INODE_BIT` is handled
above, so no need to duplicate the check.

Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20250224123747.1387122-2-hsiangkao@linux.alibaba.com
Stable-dep-of: b44686c8391b ("erofs: fix large fragment handling")
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>