[PATCH 0/1] vfs: umount_begin BKL pushdown v2

[Alessio Igor Bogani]

Push the BKL acquisition from vfs to every specific filesystems
with hope that it can be eliminated in a second moment.

Filesystems, which support umount_begin(), changed by this patch
are 9p, nfs, cifs and fuse. 

Changes:
Collapsed all patches into only one as requested by Al Viro.

Moved CIFS_SB() down into BKL protected zone as requested by Matthew Wilcox.

It is hard to say if tcon (into cifs's umount_begin() function) should be
protected by BKL. Up to now umount_begin() is always called with BKL held
so in uncertainty I maintain same logic. So I moved unlock_kernel() to at
the bottom of that function as requested by Al Viro and Matthew Wilcox.

Notes:
About cifs's umount_begin() function I suspect that a deadlock or circular
locking dependency can happens into kill-the-BKL tree if that patch is applied.
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