Btrfs: fix up bounds checking in lseek

An user reported this, it is because that lseek's SEEK_SET/SEEK_CUR/SEEK_END
allow a negative value for @offset, but btrfs's SEEK_DATA/SEEK_HOLE don't
prepare for that and convert the negative @offset into unsigned type,
so we get (end < start) warning.

[ 1269.835374] ------------[ cut here ]------------
[ 1269.836809] WARNING: CPU: 0 PID: 1241 at fs/btrfs/extent_io.c:430 insert_state+0x11d/0x140()
[ 1269.838816] BTRFS: end < start 4094 18446744073709551615
[ 1269.840334] CPU: 0 PID: 1241 Comm: a.out Tainted: G        W      3.16.0+ #306
[ 1269.858229] Call Trace:
[ 1269.858612]  [<ffffffff81801a69>] dump_stack+0x4e/0x68
[ 1269.858952]  [<ffffffff8107894c>] warn_slowpath_common+0x8c/0xc0
[ 1269.859416]  [<ffffffff81078a36>] warn_slowpath_fmt+0x46/0x50
[ 1269.859929]  [<ffffffff813b0fbd>] insert_state+0x11d/0x140
[ 1269.860409]  [<ffffffff813b1396>] __set_extent_bit+0x3b6/0x4e0
[ 1269.860805]  [<ffffffff813b21c7>] lock_extent_bits+0x87/0x200
[ 1269.861697]  [<ffffffff813a5b28>] btrfs_file_llseek+0x148/0x2a0
[ 1269.862168]  [<ffffffff811f201e>] SyS_lseek+0xae/0xc0
[ 1269.862620]  [<ffffffff8180b212>] system_call_fastpath+0x16/0x1b
[ 1269.862970] ---[ end trace 4d33ea885832054b ]---

This assumes that btrfs starts finding DATA/HOLE from the beginning of file
if the assigned @offset is negative.

Also we add alignment for lock_extent_bits 's range.
Reported-by: NToralf Förster <toralf.foerster@gmx.de>
Signed-off-by: NLiu Bo <bo.li.liu@oracle.com>
Signed-off-by: NChris Mason <clm@fb.com>

fs/btrfs/file.c

@@ -2618,23 +2618,28 @@ static int find_desired_extent(struct inode *inode, loff_t *offset, int whence)
 	struct btrfs_root *root = BTRFS_I(inode)->root;
 	struct extent_map *em = NULL;
 	struct extent_state *cached_state = NULL;
-	u64 lockstart = *offset;
-	u64 lockend = i_size_read(inode);
-	u64 start = *offset;
-	u64 len = i_size_read(inode);
+	u64 lockstart;
+	u64 lockend;
+	u64 start;
+	u64 len;
 	int ret = 0;
 
-	lockend = max_t(u64, root->sectorsize, lockend);
+	if (inode->i_size == 0)
+		return -ENXIO;
+
+	/*
+	 * *offset can be negative, in this case we start finding DATA/HOLE from
+	 * the very start of the file.
+	 */
+	start = max_t(loff_t, 0, *offset);
+
+	lockstart = round_down(start, root->sectorsize);
+	lockend = round_up(i_size_read(inode), root->sectorsize);
 	if (lockend <= lockstart)
 		lockend = lockstart + root->sectorsize;
-
 	lockend--;
 	len = lockend - lockstart + 1;
 
-	len = max_t(u64, len, root->sectorsize);
-	if (inode->i_size == 0)
-		return -ENXIO;
-
 	lock_extent_bits(&BTRFS_I(inode)->io_tree, lockstart, lockend, 0,
 			 &cached_state);