ext3: Fix oops in ext3_try_to_allocate_with_rsv()

Block allocation is called from two places: ext3_get_blocks_handle() and
ext3_xattr_block_set(). These two callers are not necessarily synchronized
because xattr code holds only xattr_sem and i_mutex, and
ext3_get_blocks_handle() may hold only truncate_mutex when called from
writepage() path. Block reservation code does not expect two concurrent
allocations to happen to the same inode and thus assertions can be triggered
or reservation structure corruption can occur.

Fix the problem by taking truncate_mutex in xattr code to serialize
allocations.

CC: Sage Weil <sage@newdream.net>
CC: stable@kernel.org
Reported-by: Fyodor Ustinov <ufm@ufm.su>
Signed-off-by: Jan Kara <jack@suse.cz>