btrfs-progs: Add further checks to btrfs replace start command

btrfs-progs: Add further checks to btrfs replace start command

Add further checks to btrfs replace start command.

The following tests where added in user space before calling
the ioctl():
1) check if the new disk is greather or equal to the old one
2) check if the source device is or a block device or a
numerical dev-id

These checks are already performed in kernel space; however
when "btrfs replace start" is ran in background is not possible
to show any error returned by the ioctl(), so in case of fail
the user had to check dmesg to understand the what happened.

Signed-off-by: Goffredo Baroncelli <kreijack@inwind.it>
Signed-off-by: David Sterba <dsterba@suse.com>