reiserfs: discard prealloc in reiserfs_delete_inode

With the removal of struct file from the xattr code,
reiserfs_file_release() isn't used anymore, so the prealloc isn't
discarded.  This causes hangs later down the line.

This patch adds it to reiserfs_delete_inode.  In most cases it will be a
no-op due to it already having been called, but will avoid hangs with
xattrs.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>