btrfs: test ENOSPC caused by many orphan items

btrfs: test ENOSPC caused by many orphan items

Btrfs has a bug where we can prematurely ENOSPC if we have lots of
orphaned files, i.e., deleted files which are still open. Add a test
which repeatedly creates and deletes a file while keeping all of the
file descriptors open. This should succeed but doesn't on Btrfs
without the fix.

[Eryu: use multi_open_unlink command to keep files open & unlinked
instead of shell scripts]

Signed-off-by: Omar Sandoval <osandov@fb.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>