loop: retry on EBUSY
loop: retry on EBUSY

If a loop device gets stolen in between getting an fd for it and setting
it up let's retry for a bit.

Closes #5246.

Signed-off-by: Christian Brauner <48455ab3070520a2d174545c7239d6d0fabd9a83@ubuntu.com>

hallyn/lxd,stgraber/lxd,hallyn/lxd,hallyn/lxd,lxc/lxd,stgraber/lxd,atxwebs/lxd,hallyn/lxd,atxwebs/lxd,hallyn/lxd,mjeanson/lxd,Skarlso/lxd,lxc/lxd,atxwebs/lxd,lxc/lxd,lxc/lxd,lxc/lxd,lxc/lxd,mjeanson/lxd,lxc/lxd,Skarlso/lxd,stgraber/lxd,mjeanson/lxd,stgraber/lxd,atxwebs/lxd,Skarlso/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd,hallyn/lxd,Skarlso/lxd