async: Don't call async_synchronize_full_special() while holding sb_lock

sync_filesystems() shouldn't be calling async_synchronize_full_special
while holding a spinlock.  The second while loop in that function is the
right place for this anyway.

Signed-off-by: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Reported-by: Grissiom <chaos.proton@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>