ocfs2: Wrap access of directory allocations with ip_alloc_sem.

OCFS2_I(inode)->ip_alloc_sem is a read-write semaphore protecting
local concurrent access of ocfs2 inodes.  However, ocfs2 directories were
not taking the semaphore while they accessed or modified the allocation
tree.

ocfs2_extend_dir() needs to take the semaphore in a write mode when it
adds to the allocation.  All other directory users get there via
ocfs2_bread(), which takes the semaphore in read mode.

Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>