proc: less memory for /proc/*/map_files readdir

dentry name can be evaluated later, right before calling into VFS.

Also, spend less time under ->mmap_sem.

Link: http://lkml.kernel.org/r/20171110163034.GA2534@avx2
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
1 file changed