.set_acl() requires a dentry, and if one isn't passed it marks the VFS
inode as not having an ACL.

This has been causing inodes with ACLs to have them "disappear" on
bcachefs filesystem, depending on which path those inodes get pulled
into the cache from.

Switching to .get_inode_acl(), like other local filesystems, fixes this.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>