vfs_shadow_copy2: add shadow_copy2_do_convert()

vfs_shadow_copy2: add shadow_copy2_do_convert()

Add a new routine shadow_copy2_do_convert() which is
like shadow_copy2_convert(), but beside calculating the
path of the snapshot file, it also returns the minimum length
of the subpath which is both inside the share and inside the
same snapshot as the file in question, i.e. (at least in the
classical case) the subdirectory of the the snapshot file's
snapshot directory that corresponds to the file's share root.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11580

Signed-off-by: Uri Simchoni <uri@samba.org>
Reviewed-by: Michael Adam <obnox@samba.org>