Revision 285021

zfs_mount(MS_REMOUNT): protect zfs_(un)register_callbacks calls

We now take z_teardown_lock as a writer to ensure that there is no I/O
while the filesystem state is in a flux.  Also, zfs_suspend_fs() ->
zfsvfs_teardown() call zfs_unregister_callbacks() and zfs_resume_fs() ->
zfsvfs_setup() call zfs_unregister_callbacks().  Previously there was no
synchronization between those calls and the calls in the re-mounting
case.  That could lead to concurrent execution and a crash.

PR:		180060
Differential Revision:	https://reviews.freebsd.org/D2865
Suggested by:	mahrens
Reviewed by:	delphij, pho, mahrens, will
MFC after:	13 days
Sponsored by:	ClusterHQ