erofs: introduce on-disk format for long xattr name prefixes

Besides the predefined xattr name prefixes, introduces long xattr name
prefixes, which work similarly as the predefined name prefixes, except
that they are user specified.

It is especially useful for use cases together with overlayfs like
Composefs model, which introduces diverse xattr values with only a few
common xattr names (trusted.overlay.redirect, trusted.overlay.digest,
and maybe more in the future).  That makes the existing predefined
prefixes ineffective in both image size and runtime performance.

When a user specified long xattr name prefix is used, only the trailing
part of the xattr name apart from the long xattr name prefix will be
stored in erofs_xattr_entry.e_name.  e_name is empty if the xattr name
matches exactly as the long xattr name prefix.  All long xattr prefixes
are stored in the packed or meta inode, which depends if fragments
feature is enabled or not.

For each long xattr name prefix, the on-disk format is kept as the same
as the unique metadata format: ALIGN({__le16 len, data}, 4), where len
represents the total size of struct erofs_xattr_long_prefix, followed
by data of struct erofs_xattr_long_prefix itself.

Each erofs_xattr_long_prefix keeps predefined prefixes (base_index)
and the remaining prefix string without the trailing '\0'.

Two fields are introduced to the on-disk superblock, where
xattr_prefix_count represents the total number of the long xattr name
prefixes recorded, and xattr_prefix_start represents the start offset of
recorded name prefixes in the packed/meta inode divided by 4.

When referring to a long xattr name prefix, the highest bit (bit 7) of
erofs_xattr_entry.e_name_index is set, while the lower bits (bit 0-6)
as a whole represents the index of the referred long name prefix among
all long xattr name prefixes.

Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Acked-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20230407141710.113882-5-jefflexu@linux.alibaba.com


Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>