Time has come to change NR_OPEN value, some production servers hit the
not so 'ridiculously high value' of 1024*1024 file descriptors per process.

This is safe to raise this value, because alloc_fd_array() uses
vmalloc() for large arrays and vmalloc() returns NULL if a too large
allocation is attempted (or in case of memory shortage)

Moore law implies this limit should be changed every 1-2 years, and it seems
we forgot doing this for ages.

Even on my old notebook, a process can open 1000000 descriptors without any 
problem.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>