fs: configfs: Factor out configfs_find_subsys_dentry()

configfs_depend_item() is quite complicated and should
be split up into smaller functions. This also allow to
share this code with other functions.

Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>