implementation of xattrop type
GF_XATTROP_ADD_DEF_ARRAY
GF_XATTROP_ADD_DEF_ARRAY64

These operations are similar to 'GF_XATTROP_ADD_ARRAY',
except that it adds a default value if xattr is missing
or its value is zero on disk.

One use-case of this operation is in inode-quota.
When a new directory is created, its default dir_count
should be set to 1. So when a xattrop performed setting
inode-xattrs, it should account initial dir_count
1 if the xattrs are not present

Here is the usage of this operation

value required in xdata for each key 
struct array {
    int32_t   newvalue_1;
    int32_t   default_1;
    int32_t   newvalue_2;
    int32_t   default_2;
    ... 
    int32_t   newvalue_n;
    int32_t   default_n;
};

xattrop GF_XATTROP_ADD_ARRAY
for i from 1 to n
{
    if (xattr (dest_i) is zero or not set in the disk)
        dest_i = dest_i + newvalue_i + default_i
    else
        dest_i = dest_i + newvalue_i
}

value in xdata after xattrop is successful
struct array {
    int32_t   dest_1;
    int32_t   dest_2;
    ...
    int32_t   dest_n;
};