Btrfs: use i_size_read() in btrfs_defrag_file()

Don't use inode->i_size directly, since we're not holding i_mutex.

This also fixes another bug, that i_size can change after it's checked
against 0 and then (i_size - 1) can be negative.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>