btrfs: add test for btrfs send with large xattrs

btrfs: add test for btrfs send with large xattrs

Verify that btrfs send is able to replicate xattrs larger than
PATH_MAX. This is possible if the b+tree leaf size is larger
than 4Kb (mkfs.btrfs's default is max(16Kb, PAGE_SIZE) as of
btrfs-progs v3.12, and max(4Kb, PAGE_SIZE in older versions).

This issue is fixed by the following linux kernel btrfs patch:

   Btrfs: send, use the right limits for xattr names and values

Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>