VFS: move mnt_want_write() and locking into lookup_open()

VFS: move mnt_want_write() and locking into lookup_open()

The mnt_want_write() call and the parent inode locking in
open_last_lookups() are only needed for lookup_open().  So we can move
them and all the got_write handling into lookup_open().

Note that we need to also check create_error when determining whether to
unlock shared or not, as O_CREAT can be cleared, but create_error is
only set of O_CREAT was set.

The fsnotify calls come too as they must be in the locked region.

Also use the existing dir_inode uniformly for dir->d_inode.

This is a step towards exporting an better "open/create" interface to nfsd.

Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jori Koolstra <jkoolstra@xs4all.nl>
Signed-off-by: NeilBrown <neil@brown.name>
Link: https://patch.msgid.link/20260714230534.776886-2-neilb@ownmail.net
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>