gfs2: Fix mmap + page fault deadlocks for buffered I/O

In the .read_iter and .write_iter file operations, we're accessing
user-space memory while holding the inode glock.  There is a possibility
that the memory is mapped to the same file, in which case we'd recurse
on the same glock.

We could detect and work around this simple case of recursive locking,
but more complex scenarios exist that involve multiple glocks,
processes, and cluster nodes, and working around all of those cases
isn't practical or even possible.

Avoid these kinds of problems by disabling page faults while holding the
inode glock.  If a page fault would occur, we either end up with a
partial read or write or with -EFAULT if nothing could be read or
written.  In either case, we know that we're not done with the
operation, so we indicate that we're willing to give up the inode glock
and then we fault in the missing pages.  If that made us lose the inode
glock, we return a partial read or write.  Otherwise, we resume the
operation.

This locking problem was originally reported by Jan Kara.  Linus came up
with the idea of disabling page faults.  Many thanks to Al Viro and
Matthew Wilcox for their feedback.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>