ext4: avoid fetching btime in ext4_getattr() unless requested

ext4: avoid fetching btime in ext4_getattr() unless requested

Linus observed that an allmodconfig build which does a lot of stat(2)
calls that ext4_getattr() was a noticeable (1%) amount of CPU time,
due to the cache line for i_extra_isize getting pulled in.  Since the
normal stat system call doesn't return btime, it's a complete waste.
So only calculate btime when it is explicitly requested.

[ Fixed to check against request_mask instead of query_flags. ]

Link: https://lore.kernel.org/r/CAHk-=wivmk_j6KbTX+Er64mLrG8abXZo0M10PNdAnHc8fWXfsQ@mail.gmail.com
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>