hfsplus: use alloc_mutex in hfsplus_sync_fs

Use alloc_mutex to protect hfsplus_sync_fs against itself and concurrent
allocations, which allows to get rid of lock_super in hfsplus.

Note that most fields in the superblock still aren't protected against
concurrent allocations, that will follow later.

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