Check for minimum partition size

On Linux block devices used for vdevs will by paritioned.  The block
device must be large enough for an 64M partition starting at offset
of 2048 sectors (part1), and a second 64M reserved partition at the
end of the device (part9).

This commit adds a capacity check when creaating the GPT label to
immediately detect a device which is too small.  With the existing
code this would be caught slightly latter when attempting to use
the partition.  Catching it sooner let's us print a more useful error.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

lib/libzfs/os/linux/libzfs_pool_os.c

@@ -273,6 +273,15 @@ zpool_label_disk(libzfs_handle_t *hdl, zpool_handle_t *zhp, const char *name)
 	vtoc->efi_parts[0].p_start = start_block;
 	vtoc->efi_parts[0].p_size = slice_size;
 
+	if (vtoc->efi_parts[0].p_size * vtoc->efi_lbasize < SPA_MINDEVSIZE) {
+		(void) close(fd);
+		efi_free(vtoc);
+
+		zfs_error_aux(hdl, dgettext(TEXT_DOMAIN, "partition would "
+		    "be less than the minimum device size (64M)"), path);
+		return (zfs_error(hdl, EZFS_LABELFAILED, errbuf));
+	}
+
 	/*
 	 * Why we use V_USR: V_BACKUP confuses users, and is considered
 	 * disposable by some EFI utilities (since EFI doesn't have a backup