erofs-utils: lib: allow xattr e_name_index as 0

erofs-utils: lib: allow xattr e_name_index as 0

Since it's implicitly supported since the initial EROFS kernel version.

It is particularly useful as an on-disk inode xattr placeholder for
on-disk core inode in-place updates, especially for root inodes because
"root_nid" recorded in the on-disk superblock is limited to 16 bits
for now.

Fixes: 1e429b74bff8 ("erofs-utils: lib: fix potential out-of-bound in xattr_entrylist()")
Reviewed-by: Sandeep Dhavale <dhavale@google.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20240729075027.712339-1-hsiangkao@linux.alibaba.com