btrfs-progs: volumes: Remove BUG_ON in raid56 write routine

btrfs-progs: volumes: Remove BUG_ON in raid56 write routine

Remove various BUG_ON in raid56 write routine, including:
1) Memory allocation error
   Old codes allocates memory when code needs new memory in a loop, and
   catch the error using BUG_ON().
   New codes allocates memory in a allocation loop first, if any failure
   is caught, freeing already allocated memories then throw -ENOMEM.

2) Write error
   Change BUG_ON() to correct return value.

3) Validation check
   Same as write error.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>