pull mnt_want_write()/mnt_drop_write() into kern_path_create()/done_path_create() resp.

One side effect - attempt to create a cross-device link on a read-only fs fails
with EROFS instead of EXDEV now.  Makes more sense, POSIX allows, etc.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>