fsopen: use square brackets around "fscontext"

Make the name of the anon inode fd "[fscontext]" instead of "fscontext".
This is minor but most core-kernel anon inode fds carry square brackets
around their name (cf. [1]). For the sake of consistency lets do the same
for the mount api:

[eventfd]
[eventpoll]
[fanotify]
[fscontext]
[io_uring]
[pidfd]
[signalfd]
[timerfd]
[userfaultfd]

Signed-off-by: Christian Brauner <christian@brauner.io>