hfsplus: introduce alloc_mutex

Use a new per-sb alloc_mutex instead of abusing i_mutex of the alloc_file
to protect block allocations.  This gets rid of lockdep nesting warnings
and prepares for extending the scope of alloc_mutex.

Signed-off-by: Christoph Hellwig <hch@tuxera.com>

diff --cc fs/hfsplus/bitmap.c
Simple merge

diff --cc fs/hfsplus/hfsplus_fs.h
Simple merge

diff --cc fs/hfsplus/super.c
Simple merge