btrfs: fix minor typo in comment

In the comment describing the sync_writers field of the btrfs_inode
struct, "fsyncing" was misspelled "fsycing."

Signed-off-by: Nathaniel Yazdani <n1ght.4nd.d4y@gmail.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>