inotify: Increase default inotify.max_user_watches limit to 1048576

The default value of inotify.max_user_watches sysctl parameter was set
to 8192 since the introduction of the inotify feature in 2005 by
commit 0eeca28300df ("[PATCH] inotify"). Today this value is just too
small for many modern usage. As a result, users have to explicitly set
it to a larger value to make it work.

After some searching around the web, these are the
inotify.max_user_watches values used by some projects:
 - vscode:  524288
 - dropbox support: 100000
 - users on stackexchange: 12228
 - lsyncd user: 2000000
 - code42 support: 1048576
 - monodevelop: 16384
 - tectonic: 524288
 - openshift origin: 65536

Each watch point adds an inotify_inode_mark structure to an inode to
be watched. It also pins the watched inode.

Modeled after the epoll.max_user_watches behavior to adjust the default
value according to the amount of addressable memory available, make
inotify.max_user_watches behave in a similar way to make it use no more
than 1% of addressable memory within the range [8192, 1048576].

We estimate the amount of memory used by inotify mark to size of
inotify_inode_mark plus two times the size of struct inode (we double
the inode size to cover the additional filesystem private inode part).
That means that a 64-bit system with 128GB or more memory will likely
have the maximum value of 1048576 for inotify.max_user_watches. This
default should be big enough for most use cases.

Link: https://lore.kernel.org/r/20201109035931.4740-1-longman@redhat.com
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Jan Kara <jack@suse.cz>

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