fs: return EAGAIN when O_NONBLOCK write should block on frozen fs

fs: return EAGAIN when O_NONBLOCK write should block on frozen fs

When user asks for O_NONBLOCK behavior for a file descriptor, return
EAGAIN instead of blocking on a frozen filesystem.

This is needed so we can fix a hang with BSD accounting on frozen
filesystems.

Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Acked-by: Joel Becker <jlbec@evilplan.org>
Cc: Mark Fasheh <mfasheh@suse.com>
Cc: Nikola Ciprich <nikola.ciprich@linuxbox.cz>
Cc: Marco Stornelli <marco.stornelli@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>