btrfs: merge blocking_writers branches in btrfs_tree_read_lock

btrfs: merge blocking_writers branches in btrfs_tree_read_lock

There are two ifs that use eb::blocking_writers. As this is a variable
modified inside and outside of locks, we could minimize number of
accesses to avoid problems with getting different results at different
times.

The access here is locked so this can only race with btrfs_tree_unlock
that sets blocking_writers to 0 without lock and unsets the lock owner.

The first branch is taken only if the same thread already holds the
lock, the second if checks for blocking writers. Here we'd either unlock
and wait, or proceed. Both are valid states of the locking protocol.

Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: David Sterba <dsterba@suse.com>