[XFS] growlock should be a mutex

m_growlock only needs plain binary mutex semantics, so use a struct mutex
instead of a semaphore for it.

SGI-PV: 968563
SGI-Modid: xfs-linux-melb:xfs-kern:29512a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>