new helpers: lock_mount_hash/unlock_mount_hash

aka br_write_{lock,unlock} of vfsmount_lock.  Inlines in fs/mount.h,
vfsmount_lock extern moved over there as well.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --cc fs/fs-writeback.c
Simple merge

diff --cc fs/internal.h
Simple merge

diff --cc fs/mount.h
Simple merge

diff --cc fs/namespace.c
Simple merge

diff --cc fs/pnode.c
Simple merge