proc_fd_getattr(): don't bother with S_ISDIR() check

that thing is callable only as ->i_op->getattr() instance and only
for directory inodes (/proc/*/fd and /proc/*/task/*/fd)

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Link: https://lore.kernel.org/20250615003321.GC3011112@ZenIV


Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>