Btrfs: Improve FL_KEEP_SIZE handling in fallocate

- We call inode_size_ok() only if FL_KEEP_SIZE isn't specified.
- As an optimisation we can skip the call if (off + len)
  isn't greater than the current size of the file. This operation
  is called under the lock so the less work we do, the better.
- If we call inode_size_ok() pass to it the correct value rather
  than a more conservative estimation.

Signed-off-by: Davide Italiano <dccitaliano@gmail.com>
Reviewed-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/file.c

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 15a09cb..7af1abd 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -2682,9 +2682,12 @@
 		return ret;
 
 	inode_lock(inode);
-	ret = inode_newsize_ok(inode, alloc_end);
-	if (ret)
-		goto out;
+
+	if (!(mode & FALLOC_FL_KEEP_SIZE) && offset + len > inode->i_size) {
+		ret = inode_newsize_ok(inode, offset + len);
+		if (ret)
+			goto out;
+	}
 
 	/*
 	 * TODO: Move these two operations after we have checked