Prefer idx_t to size_t in xattrs.c

Prefer idx_t to size_t in xattrs.c

* src/xattrs.c (xattr_map_free, xattr_map_add)
(xheader_xattr_add, xattr_map_copy, struct xattrs_mask_map)
(fixup_extra_acl_fields, xattrs_acls_cleanup, acls_get_text)
(xattrs__acls_get_a, xattrs__acls_get_d, acls_one_line)
(mask_map_realloc, xattrs_xattrs_get, xattrs__fd_set)
(xattrs_matches_mask, xattrs_xattrs_set, xattrs_print_char)
(xattrs_print): Prefer idx_t to size_t.