dcache: convert dentry flag macros to enum

dcache: convert dentry flag macros to enum

Commit 9748cb2dc393 ("VFS: repack DENTRY_ flags.") changed the value of
DCACHE_MOUNTED, which broke drgn's path_lookup() helper. drgn is forced
to hard-code it because it's a macro, and macros aren't preserved in
debugging information by default.

Enums, on the other hand, are included in debugging information. Convert
the DCACHE_* flag macros to an enum so that debugging tools like drgn
and bpftrace can make use of them.

Link: https://github.com/osandov/drgn/blob/2027d0fea84d74b835e77392f7040c2a333180c6/drgn/helpers/linux/fs.py#L43-L46
Signed-off-by: Omar Sandoval <osandov@fb.com>
Link: https://lore.kernel.org/177665a082f048cf536b9cd6af467b3be6b6e6ed.1744141838.git.osandov@fb.com
Signed-off-by: Christian Brauner <brauner@kernel.org>