zpool_vdev_remove() should handle EALREADY error return

[mmaybee]

System information

Type	Version/Name
Distribution Name	Ubuntu
Distribution Version	20.04
Kernel Version	5.4.0-150
Architecture	x86_64
OpenZFS Version	zfs-2.1.99-1

Describe the problem you're observing

The zpool_vdev_remove() library interface has explicit code to handle an EBUSY error, indicating that a device removal is already in progress, returned from the kernel. This error is remapped to EZFS_BUSY. But it is also possible that the kernel will return EALREADY under similar circumstances (the device requested for removal is already in progress of being removed). When this happens, the function will remap the EALREADY into EZFS_UNKNOWN. It would be better to map this error to something like EZFS_BUSY as well.