[XFS] avoid memory allocations in xfs_fs_vcmn_err

xfs_fs_vcmn_err can be called under a spinlock, but does a sleeping memory
allocation to create buffer for it's internal sprintf.  Fortunately it's
the only caller of icmn_err, so we can merge the two and have one single
static buffer and spinlock protecting it.  While we're at it make sure
we proper __attribute__ format annotations so that the compiler can detect
mismatched format strings.

Reported-by: Alexander Beregalov <a.beregalov@gmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Eric Sandeen <sandeen@sandeen.net>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>