readdir: Introduce dirent_size()

readdir: Introduce dirent_size()

In several places in readdir.c there are calculations of the total size
of a dirent, which contains a few fixed fields plus a name field with
variable size. To add fun every dirent is of a slightly different type:
- struct old_linux_dirent
- struct linux_dirent
- struct linux_dirent64
- struct compat_old_linux_dirent
- struct compat_linux_dirent

Replace ugly size calculation by a macro called dirent_size() which
calculates the size of the structure based on the pointed type and
the name field len.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Christophe Leroy (CS GROUP) <chleroy@kernel.org>
Link: https://patch.msgid.link/c20d2f8f6817a39401155cfc80f0dff88df116e0.1774350128.git.chleroy@kernel.org
Reviewed-by: David Laight <david.laight.linux@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>