hpfs: fix warnings when the filesystem fills up

This patch fixes warnings due to missing lock on write error path.

  WARNING: at fs/hpfs/hpfs_fn.h:353 hpfs_truncate+0x75/0x80 [hpfs]()
  Hardware name: empty
  Pid: 26563, comm: dd Tainted: P           O 3.9.4 #12
  Call Trace:
    hpfs_truncate+0x75/0x80 [hpfs]
    hpfs_write_begin+0x84/0x90 [hpfs]
    _hpfs_bmap+0x10/0x10 [hpfs]
    generic_file_buffered_write+0x121/0x2c0
    __generic_file_aio_write+0x1c7/0x3f0
    generic_file_aio_write+0x7c/0x100
    do_sync_write+0x98/0xd0
    hpfs_file_write+0xd/0x50 [hpfs]
    vfs_write+0xa2/0x160
    sys_write+0x51/0xa0
    page_fault+0x22/0x30
    system_call_fastpath+0x1a/0x1f
Signed-off-by: Mikulas Patocka <mikulas@artax.karlin.mff.cuni.cz>
Cc: stable@kernel.org  # 2.6.39+
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/hpfs/file.c

@@ -109,10 +109,14 @@ static void hpfs_write_failed(struct address_space *mapping, loff_t to)
 {
 	struct inode *inode = mapping->host;
 
+	hpfs_lock(inode->i_sb);
+
 	if (to > inode->i_size) {
 		truncate_pagecache(inode, to, inode->i_size);
 		hpfs_truncate(inode);
 	}
+
+	hpfs_unlock(inode->i_sb);
 }
 
 static int hpfs_write_begin(struct file *file, struct address_space *mapping,