reiserfs: don't preallocate blocks for extended attributes

Most extended attributes will fit in a single block.  More importantly,
we drop the reference to the inode while holding the transaction open
so the preallocated blocks aren't released.  As a result, the inode
may be evicted before it's removed from the transaction's prealloc list
which can cause memory corruption.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Jan Kara <jack@suse.cz>